riscv: dts: sifive: Sync hifive-unleashed-a00 dts from linux

This sync has changes required to use GPIO in U-Boot and
U-Boot SPL.

Sync dts from linux v5.7-rc2 commit:
"riscv: dts: Add GPIO reboot method to HiFive Unleashed DTS file"
(sha1: 0a91330b2af9f71ceeeed483f92774182b58f6d9)

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com>