pinctrl: Kconfig: fix missing include of rockchip/Kconfig

After the merge of the new, generic pinctrl-code, the include for
rockchip/Kconfig was missing.  Add it here, so we can select the
pinctrl-driver for SPL.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>