dts: wch: fix ngpios for some WCH SoCs

The CH32V20x and CH32V30x SoCs have 16 pins per GPIO bank, but in the
devicetree, `ngpios` was incorrectly set to 8.
Fix the devicetrees by setting the correct value.

Signed-off-by: Miguel Gazquez <miguel.gazquez@bootlin.com>