firmware: arm_scmi: Add support for debug counter decrement

Introduce a new `scmi_dec_count` helper to complement the existing
`scmi_inc_count`, allowing controlled decrement of SCMI debug counters.

This provides symmetry in debug counter management and enables
accurate tracking of counters that may both increase and decrease,
such as those used for in-flight message tracking.

Only active when CONFIG_ARM_SCMI_DEBUG_COUNTERS is enabled.

Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Philip Radford <philip.radford@arm.com>
Message-Id: <20250630105544.531723-2-philip.radford@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>