powerpc: Split __SYSFS_SPRSETUP macro

Split the __SYSFS_SPRSETUP macro into two parts so that registers requiring
custom read and write functions can use common code for their show and store
functions.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>