powerpc/crash: use generic crashkernel reservation

powerpc/crash: use generic crashkernel reservation

Commit 0ab97169aa05 ("crash_core: add generic function to do reservation")
added a generic function to reserve crashkernel memory.  So let's use the
same function on powerpc and remove the architecture-specific code that
essentially does the same thing.

The generic crashkernel reservation also provides a way to split the
crashkernel reservation into high and low memory reservations, which can
be enabled for powerpc in the future.

Along with moving to the generic crashkernel reservation, the code related
to finding the base address for the crashkernel has been separated into
its own function name get_crash_base() for better readability and
maintainability.

Link: https://lkml.kernel.org/r/20250131113830.925179-8-sourabhjain@linux.ibm.com
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Reviewed-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Acked-by: Hari Bathini <hbathini@linux.ibm.com>
Cc: Baoquan he <bhe@redhat.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>