imx6: Add imx6_src_get_boot_mode

imx6: Add imx6_src_get_boot_mode

For i.MX6, the bootmode determine code is part of spl_boot_device,
but there is might be a possibility for other part the code need to
check the desired boot mode for adding new functionalities like
modeboot env variable, or changing boot order etc.

So introduced imx6_src_get_boot_mode which actually reading the
boot mode register for desired modes.

More cleanup will be add in future patches.

Cc: Stefano Babic <[email protected]>
Cc: Tim Harvey <[email protected]>
Cc: Matteo Lisi <[email protected]>
Cc: Michael Trimarchi <[email protected]>
Signed-off-by: Jagan Teki <[email protected]>
Reviewed-by: Stefano Babic <[email protected]>