ArmPlatformPkg/PlatformPeim: allow PlatformPeiLib to set the boot mode

The current interdepencies between the PrePeiCore SEC module, the
platform PEIM and ArmPlatformLib is a bit awkward: due to the fact
that ArmPlatformLib is also used by SEC modules, we cannot use PEI
specific facilities in the implementation of ArmPlatformGetBootMode.
However, given that we call that library function /after/ invoking
PlatformPeiLib, there is no way for that library to set the boot mode
other than resorting to tricks like notification callbacks on arbitrary
unrelated events.

ArmPlatformLib should probably be phased out anyway, given its quirky
nature, but for now, let's fix this particular issue by deferring the
call to PlatformPeim() to after the point where we set the boot mode
by calling ArmPlatformGetBootMode ().

While we're at it, clean up the code slightly by using PeiServicesLib
instead of doing double pointer dereferencing.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>