i2c/nrf5: Implement NRF5 I2C driver.

Basic interrupt driven i2c driver for the NRf5 series.

The NRf5 series hardware imposes a constraint that various peripherals
cannot be used simultaneously.  This driver does not implement mutual
exclusion against SPI, or SPIs drivers (yet).

Change-Id: I308d93bc486eef95db7b95c6b7c9beafc0d7e97f
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>