powerpc: fix 8xx and 82xx type-punning warnings with GCC 4.7

C99's strict aliasing rules are insane to use in low-level code such as a
bootloader, but as Wolfgang has rejected -fno-strict-aliasing in the
past, add a union so that 16-bit accesses can be performed.

Compile-tested only.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Wolfgang Denk <wd@denx.de>