sunxi: Fix old GMAC pinmux setup

Commit 5bc4cd05 ("sunxi: move non-essential code out of s_init()")
moved the call to eth_init_board() from s_init() into board_init_f().
This means it's now only called from the SPL, which makes sense for
most of the other moved low-level functions. However the GMAC pinmux and
clock setup in eth_init_board() was not happy about that, so it broke
the sun7i GMAC.

Since Ethernet is of no use in the SPL anyway, just move the call into
board_init(), which is only run in U-Boot proper.

This fixes Ethernet operation for the A20 SoCs, which broke in
v2022.04-rc1, with the above mentioned commit.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Tested-by: Petr Štetiar <ynezz@true.cz> [a20-olinuxino-lime2]