riscv: dts: Add DT support for SiFive FU540 GPIO driver

Add the gpio DT node in SiFive FU540 soc-specific DT file.
Enable the gpio node in HiFive Unleashed board-specific DT file.

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>