dra7xx_evm: Enabled UART-boot mode and add dra7xx_evm_uart3 build

UART booting is supported on this SoC, but via UART3 rather than UART1.
Because of this we must change the board to use UART3 for all console
access (only one UART is exposed on this board and a slight HW mod is
required to switch UARTs).

Signed-off-by: Minal Shah <minal.shah@ti.com>
[trini: Make apply to mainline, reword commit]
Signed-off-by: Tom Rini <trini@ti.com>