uart: add baudrate and clock freqency to uart_init

Adds baudrate and clock frequency as agruments to uart_init().

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
plat-vexpress: add juno flavor

Adds support for ARM Versatile Express V2M-Juno by adding flavor juno.

UART3 (SoC UART1) is used as console uart instead of UART1 (FPGA UART1)
which is used by TSP in ARM Trusted Firmware. UART3 is used for OP-TEE
since that uart is easily accessible on the Juno board, while UART0 and
UART1 only are accessible via headers on the motherboard.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (Juno and FVP)
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>