riscv: dts: sifive unleashed: Add PWM controlled LEDs

riscv: dts: sifive unleashed: Add PWM controlled LEDs

This adds the 4 PWM controlled green LEDs to the HiFive Unleashed device
tree. The schematic doesn't specify any special function for the LEDs,
so they're added here without any default triggers and named d1, d2, d3
and d4 just like in the schematic.

Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20221012110928.352910-1-emil.renner.berthing@canonical.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>