test: gpio: Add tests for the managed API

Add a test to verify that GPIOs can be acquired/released using the managed
API. Also check that the GPIOs are released when the consumer device is
removed.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>