drivers: i2c: nrfx_twim: enable/disable when resuming/suspending

The driver already implemented some sort of runtime PM by
unconditionally calling nrfx_twim_disable/enable. Perform the same
operations in the PM callback instead of doing a full init/deinit every
time.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>