riscv: dts: jh7110: Add a gpio-restart node

Add gpio-restart node to do reset.

Before applied this patch, System Reset Extension doesn't appear with
sbi command.

OpenSBI 1.3
Machine:
  Vendor ID 489
  Architecture ID 8000000000000007
  Implementation ID 4210427
Extensions:
  sbi_set_timer
  sbi_console_putchar
...[snip]...
  IPI Extension
  RFENCE Extension
  Hart State Management Extension
  Performance Monitoring Unit Extension

After applied this patch, System Reset Extension is supported from SBI.

OpenSBI 1.3
Machine:
  Vendor ID 489
  Architecture ID 8000000000000007
  Implementation ID 4210427
Extensions:
  sbi_set_timer
  sbi_console_putchar
...[snip]...
  IPI Extension
  RFENCE Extension
  Hart State Management Extension
  System Reset Extension
  Performance Monitoring Unit Extension

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>