board/BuR/common: move I2C initialization from common part to board-specific

At this time I2C and responsible pin-mux is setup during PMIC initialziation
within common.c, this is possible because today PMIC is always connected on
I2C0.

In Future this will be changed, PMIC isn't anymore connected to bus0 in call
cases.

So we do following:
- rename enable_i2c_pin_mux0 to enable_i2c_pin_mux to be generic for enabling
  pin-mux on different or more busses.
- move the call to i2c_pin_mux and i2c_init from common.c to the specific
  board.c

Signed-off-by: Hannes Petermaier <hannes.petermaier@br-automation.com>
Signed-off-by: Hannes Petermaier <oe5hpm@oevsv.at>