arch-power: Refactor special purpose registers

arch-power: Refactor special purpose registers

This converts the definitions of all the currently defined
Special Purpose Registers (SPRs) to miscellaneous registers
which allows us to print the corresponding SPR name in the
debug logs rather than an ambiguous register number making
things much easier to correlate.

Change-Id: I03f10e3a44a8437beec453dfae2207d71ce43c1e
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>