powerpc/8xx: Drop CONFIG_8xx_COPYBACK option

powerpc/8xx: Drop CONFIG_8xx_COPYBACK option

[ Upstream commit d3efcd38c0b99162d889e36a30425345a18edb33 ]

CONFIG_8xx_COPYBACK was there to help disabling copyback cache mode
for debuging hardware. But nobody will design new boards with 8xx now.

All 8xx platforms select it, so make it the default and remove
the option.

Also remove the Mx_RESETVAL values which are pretty useless and hide
the real value while reading code.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/bcc968cda075516eb76e2f25e09821f582c566b4.1589866984.git.christophe.leroy@csgroup.eu
Signed-off-by: Sasha Levin <sashal@kernel.org>