On 85xx we don't build in dcr support because the core doesn't implement the
instructions.  This caused problems when building an 85xx kernel.  Additionally
made it so we only build __mtdcr/__mfdcr if we are CONFIG_PPC_DCR_NATIVE.

The 85xx build issue wasPointed out by Dai Haruki.
Signed-off-by: NKumar Gala <galak@kernel.crashing.org>