docs: qemu: Add instructions to boot using FIP image

Update qemu documentation with instructions to boot using FIP image.
Also, add option to build TF-A with TBBR and firmware encryption
enabled.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Change-Id: Ib3af485d413cd595352034c82c2268d7f4cb120a
1 file changed