doc: Fix minor documentation issues

1- Align steps correctly in UEFI Secure Boot process.
2- Remove width property Secure Flash layout and Secure Firmware
   Update diagrams.
3- Rename `Armv8-R64` to `Armv8-R AArch64`.

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