allwinner: sun50i_h6: improve I2C setup

Drop the unnecessary check for the I2C pins being already configured as
I2C pins (we actually don't care).
Also avoid resetting *every* peripheral that is covered by the PRCM reset
controller, instead just clear the one line connected to the I2C controller.

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