Introduce support for LINFlex driver, based on:
- the version of Freescale LPUART driver after commit b3e3bf2ef2c7 ("Merge
  4.0-rc7 into tty-next");
- commit abf1e0a98083 ("tty: serial: fsl_lpuart: lock port on console
  write").
In this basic version, the driver can be tested using initramfs and relies
on the clocks and pin muxing set up by U-Boot.

Remarks concerning the earlycon support:

- LinFlexD does not allow character transmissions in the INIT mode (see
  section 47.4.2.1 in the reference manual[1]). Therefore, a mutual
  exclusion between the first linflex_setup_watermark/linflex_set_termios
  executions and linflex_earlycon_putchar was employed and the characters
  normally sent to earlycon during initialization are kept in a buffer and
  sent afterwards.

- Empirically, character transmission is also forbidden within the last 1-2
  ms before entering the INIT mode, so we use an explicit timeout
  (PREINIT_DELAY) between linflex_earlycon_putchar and the first call to
  linflex_setup_watermark.

- U-Boot currently uses the UART FIFO mode, while this driver makes the
  transition to the buffer mode. Therefore, the earlycon putchar function
  matches the U-Boot behavior before initializations and the Linux behavior
  after.

[1] https://www.nxp.com/webapp/Download?colCode=S32V234RM

Signed-off-by: Stoica Cosmin-Stefan <redacted>
Signed-off-by: Adrian.Nitu <redacted>
Signed-off-by: Larisa Grigore <redacted>
Signed-off-by: Ana Nedelcu <redacted>
Signed-off-by: Mihaela Martinas <redacted>
Signed-off-by: Matthew Nunez <redacted>
[stefan-gabriel.mirea@nxp.com: Reduced for upstreaming and implemented
                               earlycon support]
Signed-off-by: Stefan-Gabriel Mirea <redacted>
---
 .../admin-guide/kernel-parameters.txt         |   6 +
 drivers/tty/serial/Kconfig                    |  15 +
 drivers/tty/serial/Makefile                   |   1 +
 drivers/tty/serial/fsl_linflexuart.c          | 956 ++++++++++++++++++
 include/uapi/linux/serial_core.h              |   3 +
 5 files changed, 981 insertions(+)
 create mode 100644 drivers/tty/serial/fsl_linflexuart.c

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 46b826fcb5ad..4d545732aadc 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -1090,6 +1090,12 @@
 			the framebuffer, pass the 'ram' option so that it is
 			mapped with the correct attributes.
 
+		linflex,<addr>
+			Use early console provided by Freescale LinFlex UART
+			serial driver for NXP S32V234 SoCs. A valid base
+			address must be provided, and the serial port must
+			already be setup and configured.