hikey: Add CONFIG_PINCTRL_SINGLE=y

This config is required to prevent errors on boot like below when using
https://github.com/96boards-hikey/linux

 hikey-mainline-rebase branch:

ssp-pl022 f7106000.spi: could not find pctldev for node /soc/pinmux@f7010000/spi0_pmx_func, deferring probe
gpiochip_add: GPIOs 480..487 (f8014000.gpio) failed to register

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>

kconfigs/hikey.conf

 CONFIG_DRM=y
+CONFIG_PINCTRL_SINGLE=y