sunxi: remove bogus mmc_pinmux_setup() prototype

Since all callers of mmc_pinmux_setup() are located after the definition
of that function, there is no need for a forward declaration (anymore?).

Remove the prototype along with its #ifdef guards.

Reviewed-by: Samuel Holland <samuel@sholland.org>
Tested-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>