[DRIVERS] rockchip pcie: remove hard dependency from rfkill-rk

The rockchip PCIe driver currently causes a link error when built without
rfkill-rk (CONFIG_RFKILL_RK). Error:
pcie-dw-rockchip.c:2263: undefined reference to `rfkill_get_wifi_power_state'

Instead of setting rfkill-rk as a dependency in Kconfig (as in
7ed6cad0, reverted),
make it an optional dependency by surrounding the rfkill-rk-specific code
with #ifdef