boards/same54-xpro: use 48 MHz GCLK6 for I2C

We can't run I2C off the 120 MHz main clock as the availiable dividers are too small.
Use the 48 MHz GCLK 6 instead which offers an appropriate frequency.

fixes #12037

boards/same54-xpro/include/periph_conf.h

@@ -131,7 +131,7 @@ static const i2c_conf_t i2c_config[] = {
         .scl_pin  = GPIO_PIN(PD, 9),
         .sda_pin  = GPIO_PIN(PD, 8),
         .mux      = GPIO_MUX_C,
-        .gclk_src = 0,
+        .gclk_src = 6,
         .flags    = I2C_FLAG_NONE
     }
 };