dt-bindings: fix sifive gpio properties

The sifive gpio IP block supports up to 32 GPIOs. Reflect that in the
interrupts property description and maxItems. Also add the standard
ngpios property to describe the number of GPIOs available on the
implementation.

Also add the "canaan,k210-gpiohs" compatible string to indicate the use
of this gpio controller in the Canaan Kendryte K210 SoC. If this
compatible string is used, do not define the clocks property as
required as the K210 SoC does not have a software controllable clock
for the Sifive gpio IP block.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>