ti_armv7_common: env: Use args_mmc in FIT loading path

The env command 'args_fit' does not define a root path, this forces us to
embed the rootfs into the FIT image. FIT images do not need to contain a
rootfs, when they do not the kernel will fall-back to the kernel argument
'root', if this is not defined the kernel will not boot. It is safe to
add this as when we do have the rootfs in FIT this argument is ignored.
As 'loadfit' is only called from the MMC boot path, use 'args_mmc' to
correctly populate 'bootargs'.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>