v1->v2:
1.Use two fields (pll_reg & bypass_reg) instead of the 2-values
  array (reg[2]).
2.Remove the "pll_info->version" and add a "pll_info->rate_multiplier".
3.Fix the coding style and add more detailed commit message.
4.Remove [4/5] and [5/5] in v1, because some problems were found in
  subsequent tests.
5.Remove unnecessary spinlock as Paul Burton and Paul Cercueil's suggestion.
6.Change my Signed-off-by from "Zhou Yanjie [off-list ref]"
  to "周琰杰 (Zhou Yanjie) [off-list ref]" because
  the old mailbox is in an unstable state.