sun7i:standby: use const char for the hexa digits string

arch/arm/mach-sun7i/pm/standby/mem_printk.c used non-const char arrays. However
gcc 4.6.3 likes to replace these by calls to memcpy. memcpy doesn't exist in
the kernel. Having it non-const char arrays is useless anyway in this
case. Therefore add "const" (for digit_string).