riscv: dts: sifive unmatched: Add PWM controlled LEDs

This adds the two PWM controlled LEDs to the HiFive Unmatched device
tree. D12 is just a regular green diode, but D2 is an RGB diode with 3
PWM inputs controlling the three different colours.

Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Ron Economos <re@w6rz.net>
Link: https://lore.kernel.org/r/20220705210143.315151-5-emil.renner.berthing@canonical.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>