mx5: Correct a warning in clock.c

This corects the warning below, obtained with my gcc 4.6 compiler.

arch/arm/cpu/armv7/mx5/libmx5.o: In function `decode_pll':
arch/arm/cpu/armv7/mx5/clock.c:94: undefined reference to `__aeabi_uldivmod'

I am not able to test this on MX5x hardware, but it does improve the
MAKEALL output for me. You may already have a similar patch, but I cannot
see it on the list.

Signed-off-by: Simon Glass <sjg@chromium.org>