firmware: smccc: Add support for Live Firmware Activation (LFA)

The Arm Live Firmware Activation (LFA) is a specification [1] to describe
activating firmware components without a reboot. Those components
(like TF-A's BL31, EDK-II, TF-RMM, secure paylods) would be updated the
usual way: via fwupd, FF-A or other secure storage methods, or via some
IMPDEF Out-Of-Bound method. The user can then activate this new firmware,
at system runtime, without requiring a reboot.
The specification covers the SMCCC interface to list and query available
components and eventually trigger the activation.

Add a new directory under /sys/firmware to present firmware components
capable of live activation. Each of them is a directory under lfa/,
and is identified via its GUID. The activation will be triggered by echoing
"1" into the "activate" file:
==========================================
/sys/firmware/lfa # ls -l . 6c*
.:
total 0
drwxr-xr-x    2 0 0         0 May 30 13:31 47d4086d-4cfe-9846-9b95-2950cbbd5a00
drwxr-xr-x    2 0 0         0 May 30 13:31 6c0762a6-12f2-4b56-92cb-ba8f633606d9
drwxr-xr-x    2 0 0         0 May 30 13:31 d6d0eea7-fcea-d54b-9782-9934f234b6e4

6c0762a6-12f2-4b56-92cb-ba8f633606d9:
total 0
--w-------    1 0        0             4096 May 30 13:31 activate
-r--r--r--    1 0        0             4096 May 30 13:31 activation_capable
-r--r--r--    1 0        0             4096 May 30 13:31 activation_pending
--w-------    1 0        0             4096 May 30 13:31 cancel
-r--r--r--    1 0        0             4096 May 30 13:31 cpu_rendezvous
-r--r--r--    1 0        0             4096 May 30 13:31 may_reset_cpu
-r--r--r--    1 0        0             4096 May 30 13:31 name
/sys/firmware/lfa/6c0762a6-12f2-4b56-92cb-ba8f633606d9 # grep . *
grep: activate: Permission denied
activation_capable:1
activation_pending:1
grep: cancel: Permission denied
cpu_rendezvous:1
may_reset_cpu:0
name:rmm
/sys/firmware/lfa/6c0762a6-12f2-4b56-92cb-ba8f633606d9 # echo 1 > activate
[ 2825.797871] Arm LFA: firmware activation succeeded.
/sys/firmware/lfa/6c0762a6-12f2-4b56-92cb-ba8f633606d9 #
==========================================

[1] https://developer.arm.com/documentation/den0147/latest/



Signed-off-by: Salman Nabi <salman.nabi@arm.com>
[Andre: add actual activation routine, cleanups]
Signed-off-by: Andre Przywara <andre.przywara@arm.com>