drivers: i2c: nrfx: Add implementation of i2c_recover_bus API function

Both nRF I2C drivers (i2c_nrfx_twi and i2c_nrfx_twim) perform the bus
recovery procedure in reaction to timeout (500 ms) of any requested
message transfer. Add implementation of the I2C API recovery function
in both these drivers so that it is also possible to execute this
procedure directly.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>