doc,Kconfig: Tweak wording of SystemReady Firmware option

The presentation of "Full Software Reference Stack" alongside
"SystemReady IR Reference Stack" suggests than the "full" stack does not
contain the SystemReady IR-aligned firmware.

Change the second option to "Arm SystemReady IR Firmware Only" to
reflect the fact that it is only building the firmware (plus other
artifacts to support its validation).

Update the documentation to match the above and add more details to
introduce the two top-level build options.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>