sunxi: remove unneeded i2c_init_board() call for U-Boot proper

The driver used for the Allwinner I2C IP is using proper DT and DM
enablement for a while: we enable the clock gate and de-assert the reset
line in the driver's probe() routine, and the pinmux setup is taken care
of by the DM framework.

This means the explicit call to the i2c_init_board() routine is not
needed for U-Boot proper. As the board_init() function in board.c is
only called for U-Boot proper, we can remove the call, something that
the comment there hinted at already.

Fix the comment for the board_init() function on the way: we were not
really doing board specific setup there. The fact that this function
is called from U-Boot proper only is probably more helpful for reasoning
about this code.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>