pinctrl: meson: add pinconf support

Adding pinconf support is necessary to enable boot from SPI
without breaking the eMMC. When booting from SPI, the ROM code
leave pull downs on the eMMC pad.

We need to set pinconf provided in DT to solve this

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
[narmstrong: added missing comma in pinctrl-meson-axg-pmx.c]