doc: Replace Secure Firmware Update method

The currently used Secure Firmware Update is over complicated
and problematic as U-Boot seems to try to update the system
twice, causing TF-M to display error messages which might
be confusing to the end user.

It also requires the user to wait for Linux to fully boot
and causes unnecessary reboot times.

Replace the currently used method with the following commands
in U-Boot:
fatload mmc 0:1 0xa2000000 fw.cap
efidebug capsule update -v 0xa2000000

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>