NRF5x: I2C EasyDMA support for NRF5x, extended API

This commit adds support for NRF52 TWI Master mode and slightly extends
existing I2C API. This is a breaking change, while mode selection needs to be
done during enabling I2C. There is one additional breaking change done because:
1) Unicore MX API design was PITA for writes
2) It is incompatible with EasyDMA

I strongly apologize to all two users who might be affected by this change.

lib/nrf/51/Makefile

@@ -37,7 +37,7 @@ ARFLAGS		= rcs
 
 OBJS += clock_common.o clock.o
 OBJS += gpio.o
-OBJS += i2c.o
+OBJS += i2c_common.o
 OBJS += ppi.o
 OBJS += rtc.o
 OBJS += radio_common.o ./radio.o