boards: arm64: mimx93_evk: doc: Add information about SOF board variant

This commit adds some additional information about the SOF variant
of the mimx93_evk board. It mostly goes into how SOF with Zephyr
is going to work using this board and some basic Jailhouse-related
concepts.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>