kernel: mfd: Add Hasivo STC8 mfd

kernel: mfd: Add Hasivo STC8 mfd

This STC8 microcontroller is used on a range of Hasivo managed switches.
It typically performs some fan/thermal control, and also has some
discrete IO hanging off of it.
The fan/thermal control is still somewhat unknown at this stage, but the
LED / gpio control has been determined as being two I2C registers which
need to be written to with a 'typical' Hasivo 0x40 execute mask set, to
change values.

Rather than having this expose the LED functionality / thermal control
directly, just represent it as an mfd, with some configurable OR'ing of
an execute-bit to certain registers (execute-bit-regs).  This way different
STC8 arrangements can hopefully be handled by devicetree configs rather
than needing new driver code.

Signed-off-by: Bevan Weiss <bevan.weiss@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21578
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>