doc: emulation: qemu-arm: add secure state steps

doc: emulation: qemu-arm: add secure state steps

Add build steps for building U-Boot in secure state with
TF-A and OP-TEE. It includes the full steps for building
OP-TEE and TF-A to use with U-Boot. Also a short description
how to invoke QEMU with enabled EL3 and EL2. EL3 (machine
option secure=on) is required to run TF-A.

Signed-off-by: Johannes Krottmayer <krotti83@proton.me>
Acked-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>