exynos: Correct use of 64-bit division

The current code is causing errors like this on my toolchains:

/usr/x86_64-pc-linux-gnu/armv7a-cros-linux-gnueabi/binutils-bin/2.22/
ld.bfd.real: failed to merge target specific data of file /usr/lib/gcc/
armv7a-cros-linux-gnueabi/4.7.x-google/libgcc.a(_divdi3.o)

Use do_div() to avoid this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>