PPC 74xx_7xx: Fix build warnings for ELPPC board

Fix:

misc.c: In function 'misc_init_r':
misc.c:210:3: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
misc.c:211:3: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
misc.c:212:3: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]

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