ARM: kexec: record parent context registers for non-crash CPUs

How we got to machine_crash_nonpanic_core() (iow, from an IPI, etc) is
not interesting for debugging a crash.  The more interesting context
is the parent context prior to the IPI being received.

Record the parent context register state rather than the register state
in machine_crash_nonpanic_core(), which is more relevant to the failing
condition.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>