powerpc: mpc85xx: Implemente workaround for CPU erratum A-007907

Core hang occurs when using L1 stashes. Workaround is to disable L1
stashes so software uses L2 cache for stashes instead.

Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Darwin Dingel <darwin.dingel@alliedtelesis.co.nz>
Cc: York Sun <york.sun@nxp.com>
[York S: Move SYS_FSL_ERRATUM_A007907 to Kconfig]
Reviewed-by: York Sun <york.sun@nxp.com>