ARM: dts: ux500: Skomer eMMC needs 300 ms power on

The partitions on the eMMC will not even appear sometimes, in the
datasheet for the Samsung KLMxGxxE4x we find that the power-on time
for a 4GB eMMC of this type is 300 ms and nowadays the block stack
is so fast so we are stressing it, and we need to specify that we
need this delay in the device tree.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>