rockchip: rk3399: fix incorrect boot-device in u-boot, spl-boot-device

On RK3399, mmc0 is eMMC and mmc1 is SD card, c.f. console:
MMC:   mmc@fe320000: 1, mmc@fe330000: 0

In arch/arm/mach-rockchip/spl-boot-order.c:board_boot_order, the
boot_device (BOOT_DEVICE_*) value is gotten from spl_node_to_boot_device
function. Said function returns BOOT_DEVICE_MMC1 for mmc0 (eMMC) and
BOOT_DEVICE_MMC2 for mmc1 (SD card).

Since the SD card controller is at mmc@fe320000, it should be associated
with BOOT_DEVICE_MMC2 and not BOOT_DEVICE_MMC1. Same applies to eMMC.

Let's fix that by swapping the two BOOT_DEVICEs.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Tested-by: Xavier Drudis Ferran <xdrudis@tinet.cat>