board_driver_i2c.c: hardcoded mask for PM register using SERCOM0

Hi!

I spent a lot of time trying to get the I2C to work on SERCOM3.

Registers didn't seem to update when configuring it, and I had changed the default defines to SERCOM3 instead of SERCOM0:

```
#define I2C_SERCOM SERCOM3
#define I2C_SERCOM_GCLK_ID GCLK_CLKCTRL_ID_SERCOM3_CORE_Val
#define I2C_SERCOM_CLK_GEN 0
#define I2C_SERCOM_APBCMASK PM_APBCMASK_SERCOM3

```
Inside the function `initClockNVIC()` there is the following line:

`PM->APBCMASK.reg |= PM_APBCMASK_SERCOM0;`

Shouldn't this line use the define `I2C_SERCOM_APBCMASK` instead of the hardcoded SERCOM0 mask for the PM register?

Cheers!

Open board_driver_i2c.c and inspect initClockNVIC(), especially the PM->APBCMASK.reg assignment and the I2C_SERCOM_APBCMASK define. Verify the clock and mask configuration when I2C_SERCOM is changed from SERCOM0 to SERCOM3. Done means the selected SERCOM's mask is used instead of a hardcoded SERCOM0 mask.

Written by the indexing model from the issue text.