arm: imx: make bmode command work with SPL/U-Boot combo

arm: imx: make bmode command work with SPL/U-Boot combo

The bmode command forces the SoC to use a specific boot device
by writing its boot mode into SRC_GPR9, and notifying the SoC of
the change using SRC_GPR10[28] bit: if the bit is on, bootROM
uses the value in SRC_GPR9 instead of SRC_SMBR1 to determine
the boot device.

SPL on the other hand is oblivious to this distinction, so once
the bootROM loads SPL from the device configured in SRC_GPR10,
SPL will attempt to load U-Boot from the device configured in
SRC_SMBR1, which is not updated by the bootROM to the value in
SRC_GPR9.

The result is that the selected boot device is not used across all
the boot stages.

Update spl_boot_device() to look at gpr9 when necessary.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Troy Kisky <troy.kisky@boundarydevices.com>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Eric Nelson <eric.nelson@boundarydevices.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Heiko Schocher <hs@denx.de>