powerpc/pseries: Fix endian issues in nvram code

The NVRAM code has a number of endian issues. I noticed a very
confused error log count:

RTAS: 100663330 -------- RTAS event begin --------

100663330 == 0x06000022. 0x6 LE error logs and 0x22 BE error logs.

The pstore code has similar issues - if we write an oops in one
endian and attempt to read it in another we get junk.

Make both of these formats big endian, and byteswap as required.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>