Add CCM_CCGR0_UART0_CTRL_MASK clock definition and add TX/RX iomux
definitions for UART0 (aka. SCI0).

Signed-off-by: Marcel Ziswiler <marcel@ziswiler.com>
[stefan@agner.ch: regrouped patch]
Signed-off-by: Stefan Agner <stefan@agner.ch>