ARM: uniphier: show STM (SCP) status on boot and pinmon command

The SCP (System Control Processor) or what we call STM (Stand-by
MPU) is integrated in LD4, Pro4, sLD8, LD6b, LD11, and LD20.
For these SoCs, show the information if STM is enabled.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>