firmware: arm_scmi: Move macros and helpers to common.h

Move message header specific macros and helper routines to common.h as
they will be used outside of driver.c in a later commit.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/6615db480370719b0a0241447a5f3feb8eea421f.1580448239.git.viresh.kumar@linaro.org
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>