arm64/boot: Factor out a macro to check SPE version

We check the version of SPE twice, and we'll add one more check in the
next commit so factor out a macro to do this. Change the #3 magic number
to the actual SPE version define (V1p2) to make it more readable.

No functional changes intended.

Tested-by: Leo Yan <leo.yan@arm.com>
Reviewed-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: James Clark <james.clark@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>