gpio: update dir_flags management

Update the flag management in GPIO uclass: the desc->flags is always
combined with the requested flags and the GPIO descriptor is updated
for further call.

Add a function dm_gpio_get_dir_flags to get dynamically
the current dir_flags (configuration and value).

This patch prepare introduction of the dir flags support with new ops.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>