soc/amd/common/gpio: Make macro names for GPIO flags consistent

soc/amd/common/gpio: Make macro names for GPIO flags consistent

`soc_amd_gpio` structure uses a flag field to store additional
information about GPIO configuration that does not end up directly in
the GPIO control register. However, the naming for these flags is not
consistent across event triggers and special configurations. This
change updates the flag names to be consistent (starting with
GPIO_FLAG_*) and adds some helper functions for GPIO events.

In the following CLs, more changes will be made to drop some of the
special flags which are not really required.

BUG=b:159944426

Change-Id: Idca795c3e594eb956d297d5ba5d08f75b5563ee5
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42869
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>