tegra: clock: Avoid a divide-by-zero error

The clock fix-up for tegra is still present in the code. It causes a
divide-by-zero bug after relocation when chain-loading U-Boot from
coreboot. Fix this by adding a check.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 74686766 (ARM: tegra: fix clock_get_periph_rate() for UART clocks)