powerpc: use consistent types in mktree

gcc v4.4 currently produces this build warning:

arch/powerpc/boot/mktree.c: In function 'main':
arch/powerpc/boot/mktree.c:104: warning: dereferencing type-punned pointer will break strict-aliasing rules

tmpbuf is only used as an array of unsigned ints, so declare it that way.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>