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 0b6a1702cbe36550d6e22ae462c82c1d5c3af37f), 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>