ux500: Add eMMC support in U5500.

U5500 now boots from sdi0 (onboard eMMC).
Change machine type to U5500.
Adjust uart and sdi0 clock rates for u5500.
All necessary clocks must be enabled before Linux starts because
there is no clock tree support in u5500 yet.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>