powerpc/85xx: Remove FSL_85XX_CACHE_SRAM

CONFIG_FSL_85XX_CACHE_SRAM is an option that is not
user selectable and which is not selected by any driver
nor any defconfig.

Remove it and all associated code.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9949813a6b758903b7bee910f798ba2ca82ff8ee.1648720908.git.christophe.leroy@csgroup.eu