powerpc/crash: Fix non-smp kexec preparation

In non-smp configurations, crash_kexec_prepare is never called in
the crash shutdown path. One result of this is that the crashing_cpu
variable is never set, preventing crash_save_cpu from storing the
NT_PRSTATUS elf note in the core dump.

Fixes: c7255058 ("powerpc/crash: save cpu register data in crash_smp_send_stop()")
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250211162054.857762-1-eajames@linux.ibm.com