board: sifive: unmatched: refine GEMGXL initialized function in SPL

Create a new function spl_reset_device_by_gpio to reset the device
whose reset pin is connected to the GPIO. Then, using this function
to initialize GEMGXL.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>