sunxi: binman: Support FIT generation for 32-bit SoCs

Some 32-bit SoCs can use SCP firmware to implement additional PSCI
functionality, such as system suspend. In order to load this firmware
from SPL, we need to generate and use a FIT instead of a legacy image.

Adjust the binman FIT definition so it does not rely on TF-A BL31, as
this is not used on 32-bit SoCs. Instead, after loading the firmware,
U-Boot proper is executed directly. This requires to provide an
entry point property for the U-Boot image node, which confuses some
loaders like sunxi-fel, so protect that to only be used if we have no
BL31.

Signed-off-by: Samuel Holland <samuel@sholland.org>
[Andre: filter U-Boot entry point property]
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>