doc: ti: document R5 falcon mode for AM62 platforms

doc: ti: document R5 falcon mode for AM62 platforms

This patch adds user documentation for R5 falcon mode for AM62
platforms. The main section is added to am62x_sk.rst and other documents
just include the relevant sections. Steps to build falcon support, usage
and the modified R5 memory map have been documented.

Two svg images have also been added for reference, one for the modified
tifalcon.bin and other for the fitImage format specific to R5 falcon
mode.

Signed-off-by: Anshul Dalal <anshuld@ti.com>