arm64: dts: hikey960: pinctrl: Fix spi2/spi3 pinconf

Only the pinmux was selected, not the pinconf, leading to spi issues.
Increase drive strength so that max speed (25Mhz) can be achieved.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>