Since commit c89c0114955a ("mfd: syscon: Set regmap max_register in
of_syscon_register") the syscon uses ioremap so it fails on COMPILE_TEST
without HAS_IOMEM:

drivers/mfd/syscon.c: In function ?of_syscon_register?:
drivers/mfd/syscon.c:67:9: error: implicit declaration of function ?ioremap? [-Werror=implicit-function-declaration]
  base = ioremap(res.start, resource_size(&res));
         ^
drivers/mfd/syscon.c:67:7: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
  base = ioremap(res.start, resource_size(&res));
       ^
drivers/mfd/syscon.c:109:2: error: implicit declaration of function ?iounmap? [-Werror=implicit-function-declaration]
  iounmap(base);

When selecting MFD_SYSCON, depend on HAS_IOMEM to avoid unmet direct
dependencies.

Reported-by: kbuild test robot <redacted>
Fixes: c89c0114955a ("mfd: syscon: Set regmap max_register in of_syscon_register")
Signed-off-by: Krzysztof Kozlowski <redacted>
---
 drivers/mfd/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index aa21dc55eb15..2e5b1e525a1d 100644
--- a/drivers/mfd/Kconfig
+++ b/drivers/mfd/Kconfig

@@ -1034,6 +1034,7 @@ config MFD_SUN6I_PRCM
 
 config MFD_SYSCON
 	bool "System Controller Register R/W Based on Regmap"
+	depends on HAS_IOMEM
 	select REGMAP_MMIO
 	help
 	  Select this option to enable accessing system control registers

-- 
2.5.0