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>

drivers/firmware/Kconfig

@@ -10,7 +10,7 @@ config ARM_PSCI_FW
 
 config ARM_SCPI_PROTOCOL
 	tristate "ARM System Control and Power Interface (SCPI) Message Protocol"
-	depends on ARM_MHU
+	depends on MAILBOX
 	help
 	  System Control and Power Interface (SCPI) Message Protocol is
 	  defined for the purpose of communication between the Application