rockchip: find U-boot proper boot device by inverting the logic that sets it

rockchip: find U-boot proper boot device by inverting the logic that sets it

BOOT_DEVICE_* is set by spl_node_to_boot_device() depending on the block
device number associated with the MMC device the SPL used to load U-Boot
proper from. It is NOT related to the mmc alias in the Device Tree.

For SPI flashes, all SPI flashes will return BOOT_DEVICE_SPI so there's
currently no way to know from which one the SPL loaded U-Boot proper
from. Therefore, let's just find the first valid candidate in
/chosen/u-boot,spl-boot-order that is a SPI flash and return that path.
This is a best effort.

While the original implementation may have worked, using the exact same
mechanism but in inverted fashion makes it less likely to have
surprising corner-cases or side-effects.

A nice side-effect is that all existing and future Rockchip SoCs now
automatically have their /chosen/u-boot,spl-boot-device set.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>