mfd: syscon: Allow reset control for syscon devices

Simple syscon devices may require deassertion of a reset signal in order
to access their register set. Rather than requiring a custom driver to
implement this, we can use the generic "resets" specifiers to link a
reset line to the syscon.

This change adds an optional reset line to the syscon device
description, and deasserts the reset if detected.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230105005010.124948-3-jk@codeconstruct.com.au