powerpc: mpc85xx: Implement CPU erratum A-007907 for secondary cores

Commit 06ad970b ("powerpc: mpc85xx: Implemente workaround for CPU
erratum A-007907") clears L1CSR2 for the boot core, but other cores
don't run through the workaround. Add similar code for secondary
cores to clear DCSTASHID field in L1CSR2 register.

Signed-off-by: York Sun <york.sun@nxp.com>