gpio/nrf5: Fix GPIO_ACCESS_BY_PIN behaviour

Adjust the implementation of gpio_pin_read() to return 0 or 1 rather
than 0 or 2^pin.  This ensures consistent behaviour with other gpio
device drivers, and conforms to the behaviour documented in gpio.h by
the previous patch.

Change-Id: Ia02aa68105a406f355d16a3f9b0550f948658483
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>