arm: juno: Fix UART clock rate

The UART base clock rate was typo-ed in the header file, probably because
the reference (the Linux .dts) was also wrong[1].

Fix the number to make the baud rate more correct.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=39a1a8941b2


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