firmware: arm_scmi: Add SMCCC discovery dependency in Kconfig

firmware: arm_scmi: Add SMCCC discovery dependency in Kconfig

ARM_SCMI_PROTOCOL depends on either MAILBOX or HAVE_ARM_SMCCC_DISCOVERY,
not MAILBOX alone. Fix the depedency in Kconfig file and driver to
reflect the same.

Link: https://lore.kernel.org/r/20210521134055.24271-1-etienne.carriere@linaro.org
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
[sudeep.holla: Minor tweaks to subject and change log]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>