powerpc/8xx: Use SPRN_SPRG_SCRATCH2 in DTLB miss exception

Use SPRN_SPRG_SCRATCH2 in DTLB miss exception instead of DAR
in order to be similar to ITLB miss exception.

This also simplifies mpc8xx_pmu_del()

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e3cc8f023ef40e1e8ae144e4dd1330a5ff022528.1606231483.git.christophe.leroy@csgroup.eu