board/evb64260/zuma_pbb_mbox.c: Fix GCC 4.6 build warnings

Fix:
zuma_pbb_mbox.c: In function 'zuma_mbox_dump':
zuma_pbb_mbox.c:115:2: warning: dereferencing type-punned pointer will
break strict-aliasing rules [-Wstrict-aliasing]
zuma_pbb_mbox.c:117:2: warning: dereferencing type-punned pointer will
break strict-aliasing rules [-Wstrict-aliasing]

Signed-off-by: Wolfgang Denk <wd@denx.de>