wilc1000: Support chip sleep over SPI

chip_allow_sleep() only supported wakeup via SDIO, which made the
driver unusable over SPI.  This code is a straight forward port from
the driver in the linux-at91 repository.

Signed-off-by: David Mosberger-Tang <davidm@egauge.net>
Acked-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210224033317.1507603-1-davidm@egauge.net