ArmPkg: Fix the return type of the ReadCCSIDR function

ArmPkg: Fix the return type of the ReadCCSIDR function

When CCIDX is supported, the Current Cache Size ID Register contains
data above 32 bits: namely the number of sets. Avoid truncating this
by returning a UINTN instead of UINT32. On AARCH32, the expanded
number of sets data can be read via the CCSIDR2 register.

Also, add Doxygen comments for the function.

Signed-off-by: Rebecca Cran <rebecca@nuviainc.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>