uboot-phytec

imx: mx7: move get_boot_device to cpu.c

Move get_boot_device to cpu.c to prepare adding i.MX8M support,
because i.MX8M share same code with i.MX7.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>