sunxi: SPL: add FIT config selector for Pine64 boards

For a board or platform to support FIT loading in the SPL, it has to
provide a board_fit_config_name_match() routine, which helps to select
one of possibly multiple DTBs contained in a FIT image.
Provide a simple function which chooses the DT name U-Boot was
configured with.
If the DT name is one of the two Pine64 versions, determine the exact
model by checking the DRAM size.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@openedev.com>