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: Kumar Gala <galak@kernel.crashing.org>