sunxi: soft-i2c: Fix gpio handling to work with the driver-model

i2c_init_board() gets called before the driver-model (gpio) code is
initialized, so move the setup of the soft-i2c pins out of i2c_init_board()
and into board_init(), at which time the driver-model setup has been done.

Also add proper error checking and properly request the gpios as that is
mandatory with the driver-model.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Ian Campbell <ijc@hellion.org.uk>