mfd: Add Congatec Board Controller driver

Add core MFD driver for the Board Controller found on some Congatec SMARC
module. This Board Controller provides functions like watchdog, GPIO, and
I2C busses.

This commit adds support only for the conga-SA7 module.

Signed-off-by: Thomas Richard <thomas.richard@bootlin.com>
Link: https://lore.kernel.org/r/20241001-congatec-board-controller-v3-1-39ceceed5c47@bootlin.com


Signed-off-by: Lee Jones <lee@kernel.org>