Silicon/Qemu: Move SbsaQemu MPIDR-retrieval function to FdtHelperLib

Commit 822634fc ("SbsaQemu: Update SbsaQemuAcpiDxe to use FdtHelperLib")
replaced the CountCpusFromFdt() function in SbsaQemuAcpiDxe with a call to
FdtHelperCountCpus() in FdtHelperLib. This ended up leaving static variables
FdtFirstCpuOffset and FdtCpuNodeSize uninitialised, such that the GetMpidr()
function kept returning the value for cpu 0.

Resolve this by moving the GetMpidr() function over to FdtHelperLib, where
it can again share these variables with FdtHelperCountCpus().

Fix up coding style issues as part of copy:
- Add m prefix to module-global variables.
- Add doxygen function comment header.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Graeme Gregory <graeme@nuviainc.com>
Cc: Radoslaw Biernacki <rad@semihalf.com>
Cc: Tanmay Jagdale <tanmay.jagdale@linaro.org>
Cc: Rebecca Cran <rebecca@nuviainc.com>
Reported-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Signed-off-by: Leif Lindholm <leif@nuviainc.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Tested-By: Graeme Gregory <graeme@nuviainc.com>
Tested-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>