gpio/rockchip: extended debounce support is only available on v2

The gpio driver runs into issues on v1 gpio blocks, as the db_clk
and the whole extended debounce support is only ever defined on v2.
So checking for the IS_ERR on the db_clk is not enough, as it will
be NULL on v1.

Fix this by adding the needed condition for v2 first before checking
the existence of the db_clk.

This caused my rk3288-veyron-pinky to enter a reboot loop when it
tried to enable the power-key as adc-key device.

Fixes: 3bcbd1a8 ("gpio/rockchip: support next version gpio controller")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>