hardware: Update pSeries entries

14.4 and 15.1 support little-endian pSeries. 15.0 has boot panic due to
bug in SLOF, but it should be reclassified as QEMU pSeries instead of
IBM.

Reviewed by: kbowling, ziaee
Signed-off-by: Minsoo Choo <minsoo@minsoo.io>
Differential Revision: https://reviews.freebsd.org/D57102