powerpc/32: Fix critical and debug interrupts on BOOKE

32 bits BOOKE have special interrupts for debug and other
critical events.

When handling those interrupts, dedicated registers are saved
in the stack frame in addition to the standard registers, leading
to a shift of the pt_regs struct.

Since commit db297c3b ("powerpc/32: Don't save thread.regs on
interrupt entry"), the pt_regs struct is expected to be at the
same place all the time.

Instead of handling a special struct in addition to pt_regs, just
add those special registers to struct pt_regs.

Fixes: db297c3b

 ("powerpc/32: Don't save thread.regs on interrupt entry")
Cc: stable@vger.kernel.org
Reported-by: Radu Rendec <radu.rendec@gmail.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/028d5483b4851b01ea4334d0751e7f260419092b.1625637264.git.christophe.leroy@csgroup.eu