[POWERPC] Only export __mtdcr/__mfdcr if CONFIG_PPC_DCR is set

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>