powerpc: change mfpvr return type to uint32_t

As the Processor Version Register (PVR) is a 32-bit PowerPC
register, change mfpvr() return type to match it and avoid
type casts on its callers.

Suggested by: jhibbits
Reviewed by: jhibbits, imp
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D31332