tegra: add CONSOLE_MUX support to tegra-kbc

Add support for CONSOLE_MUX to tegra-kbc driver.  This requires
adding a flag to struct keyb to know the driver has already been
initialized so if we try to initialize it again we can just return
success.  Also call into iomux_doenv() from drv_keyboard_init to
re-evaluate the stdin string.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>