powerpc/fsl-booke: Use SPRN_SPRGn rather than mfsprg/mtsprg

powerpc/fsl-booke: Use SPRN_SPRGn rather than mfsprg/mtsprg

This fixes a build break that was probably introduced with the removal
of -Wa,-me500 (commit f49596a4cf4753d13951608f24f939a59fdcc653), where
the assembler refuses to recognize SPRG4-7 with a generic PPC target.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Dongsheng Wang <dongsheng.wang@freescale.com>
Cc: Anton Vorontsov <avorontsov@mvista.com>
Reviewed-by: Wang Dongsheng <dongsheng.wang@freescale.com>
Tested-by: Wang Dongsheng <dongsheng.wang@freescale.com>