firmware: arm_scpi: make it depend on MAILBOX instead of ARM_MHU

ARM_SCPI_PROTOCOL can be used with any mailbox and not just ARM MHU
mailbox controller. This patch drops it's dependency on ARM_MHU and
make it depend on just mailbox framework.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>