Fix compiler warnings for EVB64260, P3G4 and ZUMA

Fix following warnings:

$ ./MAKEALL EVB64260 P3G4 ZUMA
Configuring for EVB64260 board...
mpsc.c: In function 'mpsc_putchar_early':
mpsc.c:121: warning: dereferencing type-punned pointer will break strict-aliasing rules
mpsc.c:127: warning: dereferencing type-punned pointer will break strict-aliasing rules
...

Signed-off-by: Anatolij Gustschin <agust@denx.de>