arm: sunxi: Correct warning in board_fit_config_name_match

When building this with clang, we get a warning about having excess
parenthesis here, or that we're incorrectly using "==" when we want "=".
Correct these by using the common size macro.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[Andre: Use SZ_512M as per Simon's suggestion]
Signed-off-by: Andre Przywara <andre.przywara@arm.com>