imx: move get_boot_device to common file

imx: move get_boot_device to common file

i.MX8MN/P/ULP supports ROM API, they have almost same get_boot_device
implementation, so move to a common file. And when support i.MX9,
no need to include the other function copy.

Since sys_proto.h is included in imx_romapi.c, there will be build
warning for i.MX8M because wdog_regs not defined, so include imx-regs.h
in i.MX8M sys_proro.h

Signed-off-by: Peng Fan <peng.fan@nxp.com>