powerpc/8xx: remove EXCEPTION_PROLOG/EPILOG_0 and change r3 to r12

EXCEPTION_PROLOG_0 and EXCEPTION_EPILOG_0 were added some
time ago in order to regroup the two mtspr/mfspr to SCRATCH0 and
SCRATCH1 and the mfcr/mtcr in order to ease entry and exit of
function not using the full EXCEPTION_PROLOG.

Since then, the mfcr/mtcr has been taken out, hence just leaving
the two mtspr/mfspr in the macro.

In order to improve readability of the exception functions, we
remove those two macros and copy back the two mtspr/mfspr instead.

As r10 and r11 are used for SCRATCH0 and SCRATCH1, lets also use
r12 for SCRATCH2. It will also improve the readability/maintenance.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>