powerpc/85xx: Add missing config option for CACHE SRAM code

fsl_85xx_l2ctlr.o and fsl_85xx_cache_sram.o are built only
if CONFIG_FSL_85XX_CACHE_SRAM is defined. The driver that
qualifies and wants to make use of the CACHE SRAM's exported
API (i.e. a freescale net driver) should (be able to) select
this config option.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>