rockchip: gpio: remove outdated/misleading comment

Remove a comment claiming that this driver only supports the RK3288,
as we also use it on the RK3368, RK3399 and (most likely) on other
variants.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

Version-changes: 2
- use the dev_read_addr_ptr function in rk_gpio.c