i2c/nrf5: Remove r/w to POWER register.

The reset code that toggles the POWER register fails to build on
NRF52.  In discussion with the Nordic folks it transpires that we
don't actually need this code.

For future reference there is a related errata here:
http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.Rev1.errata/anomaly_832_89.html?cp=2_2_1_0_1_26



Change-Id: I2ba8c2bb814f8f95ff3f340862c75a8bc437c573
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>