firmware: Google VPD sysfs driver

This patch introduces the Google Vital Product Data driver.

This driver reads Vital Product Data from coreboot tables and then
creates the corresponding sysfs entries under /sys/firmware/vpd to
provide easy access for userspace programs (does not require flashrom).

The sysfs is structured as follow:

 /sys/firmware/vpd
 |-- ro
 |   |-- key1
 |   `-- key2
 |-- ro_raw
 |-- rw
 |   `-- key1
 `-- rw_raw

Where ro_raw and rw_raw contain the raw VPD partition. The files under
ro and rw correspond to the key name in the VPD and the the file content
is the value for the key.

Signed-off-by: Wei-Ning Huang <wnhuang@google.com>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>