[POWERPC] 85xx/86xx: refactor RSTCR reset code

On the majority of 85xx & 86xx we have a register that's ability to
assert HRESET_REQ to reset the board.  We refactored that code so it
can be shared between both platforms into fsl_soc.c and removed all
the duplication in each platform directory.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>