arm: dts: u8500: u-boot: Add fixed clock for eMMC

So far there is no need for a clock driver in U-Boot because the
previous boot stage leaves all the necessary clocks on. However,
some drivers in U-Boot (e.g. arm_pl180_mmci) depend on having a clock
driver to obtain the clock frequency.

Setting up the clock drivers properly is a bit tricky on U8500,
so for now add a simple fixed-clock for the eMMC that allows obtaining
the clock frequency. This should be replaced eventually if some board
actually requires enabling some of the clocks.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>