ARM: ux500: move UART pin control to the device tree

This moves the static, device-tied pin control configuration
out of the board file board-mop500-pins.c and into the device
tree.

We create a new .dtsi-file to be shared between all the
MOP500-related boards, that include all HREF variants and
the Snowball board. Assign pin states for HREF and Snowball
boards alike.

Cc: Lee Jones <lee.jones@linaro.org>
Cc: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>