powerpc/kdump: Use ppc_save_regs() in crash_setup_regs()

The patch replaces internal registers dump implementation with
ppc_save_regs(). From now on PPC64 and PPC32 are using the same
code for crash_setup_regs().

NOTE: The old regs dump implementation was capturing SP (r1) directly
as is, so you could see crash_kexec() function on top of the back-trace.
But ppc_save_regs() goes up one stack frame, so you'll not see it
anymore, at the top-level you'll see who actually triggered the crash
dump instead.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>