dm: omap: gpio: Support driver model

Add driver model support to this driver, while retaining support for the
legacy system. Driver model GPIO support is enabled with CONFIG_DM_GPIO
as usual.

Since gpio_is_valid() no longer exists, we can use the -EINVAL error
returned from gpio_request().

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>