docs(juno): update TF-A build instructions

Clean up instructions for building/running TF-A on the
Juno platform and add correct link to SCP binaries.

Signed-off-by: Zelalem Aweke <zelalem.aweke@arm.com>
Change-Id: I536f98082e167edbf45f29ca23cc0db44687bb3b
1 file changed