sunxi: board: Simplify Pine A64 DT selection logic

sunxi: board: Simplify Pine A64 DT selection logic

Instead of using an entirely separate matching algorithm, simply update
the name of the DT we want to match. Enabling this logic does not depend
on the FIT config name, only on the initial guess of the board name.

Importantly, the initial guess must be "sun50i-a64-pine64-plus", because
otherwise the logic would trigger when "sun50i-a64-pine64-lts" was
written to the SPL header.

Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>