platform/x86: Add Slim Bootloader firmware update signaling driver

Slim Bootloader(SBL) is a small open-source boot firmware,
designed for running on certain Intel platforms. SBL can be
thought-of as fulfilling the role of a minimal BIOS
implementation, i.e initializing the hardware and booting
Operating System.

Since SBL is not UEFI compliant, firmware update cannot be triggered
using standard UEFI runtime services. Further considering performance
impact, SBL doesn't look for a firmware update image on every reset
and does so only when firmware update signal is asserted.

SBL exposes an ACPI-WMI device which comes up in sysfs as
/sys/bus/wmi/44FADEB1xxx and this driver adds a
"firmware_update_request" device attribute. This attribute normally
has a value of 0 and userspace can signal SBL to update firmware,
on next reboot, by writing a value of 1 like:

echo 1 > /sys/bus/wmi/devices/44FADEB1xxx/firmware_update_request

This driver only implements a signaling mechanism, the actual firmware
update process and various details like firmware update image format,
firmware image location etc are defined by SBL and are not in the
scope of this driver.

DocLink: https://slimbootloader.github.io/security/firmware-update.html


Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>