arm64:mach-k3 am625_init: Correct boot mode detection

The boot mode detection assumes that BOOT_DEVICE_MMC2 should always
result in MMCSD_MODE_FS, but MMCSD_MODE_RAW is also a valid option for
this port.

The current logic also avoids looking at the bootmode pin strapping,
which should be the primary means of determining whether a device is
being booted in MMCSD_MODE_EMMCBOOT mode.

Switch around the logic to check the boot mode to determine whether the
eMMC boot mode is expected or MMC/SD boot mode. From there we can look
at the boot mode config if in MMC/SD boot mode to determine whether to
attempt RAW or FS based booting.

This change allows U-Boot to also be successfully booted from RAW
offsets in addition to from a filesystem.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>