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 <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>