imx: ventana: gsc: add gsc sleep command

imx: ventana: gsc: add gsc sleep command

The Gateworks System Controller on Ventana boards has the ability to
disable the board's primary power supply until the RTC hits a specific
time. When sleeping a button-down event on the GSC user pushbutton will
wake the board before it's wake time has been reached. This feature is
referred to as GSC sleep.

Add a command to invoke sleep mode for a specified number of seconds.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>