rockchip: spl-boot-order: Defer probe of boot device

rockchip: spl-boot-order: Defer probe of boot device

Boot devices are being probed when SPL boot order is determined. This
may delay boot slightly and can prevent booting from SPI Flash on boards
that use same pins for SPI Flash and eMMC due to pinctrl being applied
prior to booting.

Instead defer probe of the boot device until SPL try to load image from
the boot device by using uclass_find_device_by_of_offset() instead of
the get variant.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>