firmware: google: Implement cbmem in sysfs driver

The CBMEM area is a downward-growing memory region used by coreboot to
dynamically allocate tagged data structures ("CBMEM entries") that
remain resident during boot.

This implements a driver which exports access to the CBMEM entries
via sysfs under /sys/bus/coreboot/devices/cbmem-<id>.

This implementation is quite versatile.  Examples of how it could be
used are given below:

* Tools like util/cbmem from the coreboot tree could use this driver
  instead of finding CBMEM in /dev/mem directly.  Alternatively,
  firmware developers debugging an issue may find the sysfs interface
  more ergonomic than the cbmem tool and choose to use it directly.

* The crossystem tool, which exposes verified boot variables, can use
  this driver to read the vboot work buffer.

* Tools which read the BIOS SPI flash (e.g., flashrom) can find the
  flash layout in CBMEM directly, which is significantly faster than
  searching the flash directly.

Write access is provided to all CBMEM regions via
/sys/bus/coreboot/devices/cbmem-<id>/mem, as the existing cbmem
tooling updates this memory region, and envisioned use cases with
crossystem can benefit from updating memory regions.

Link: https://issuetracker.google.com/239604743
Cc: Stephen Boyd <[email protected]>
Cc: Tzung-Bi Shih <[email protected]>
Reviewed-by: Guenter Roeck <[email protected]>
Reviewed-by: Julius Werner <[email protected]>
Tested-by: Jack Rosenthal <[email protected]>
Signed-off-by: Jack Rosenthal <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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