powerpc/mpc8xxx: Move HWCONFIG_BUFFER_SIZE into config.h

Before proper environment is setup, we extract hwconfig and put it into a
buffer with size HWCONFIG_BUFFER_SIZE. We need to enlarge the buffer to
accommodate longer string. Since this macro is used in multiple files, we
move it into arch/powerpc/include/asm/config.h.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>