arm: am33xx: Make pin multiplexing functions optional

This patch provides default implementations of the two functions
set_uart_mux_conf and set_mux_conf_regs. Hence boards not using
them do not need to provide their distinct empty definitions.

Signed-off-by: Felix Brack <fb@ltec.ch>
Reviewed-by: Tom Rini <trini@konsulko.com>