doc: imx: psb: Document usage of SRC_GPR10 PERSIST_SECONDARY_BOOT for A/B switching

Document SRC_GPR10 PERSIST_SECONDARY_BOOT functionality. This is useful for
reliable bootloader A/B updates, as it permits switching between two copies
of bootloader at different offsets of the same storage. The switch happens
in case one copy is corrupted OR can be enforced by user. This functionality
is present at least since i.MX53, however is poorly documented in all known
SoC datasheets, hence this document aims to clarify the usage, currently on
i.MX7D and i.MX8MM.

Signed-off-by: Marek Vasut <marex@denx.de> # Original MX7D work, this document
Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io> # All the MX8M work
Cc: Christoph Niedermaier <cniedermaier@dh-electronics.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Harald Seiler <hws@denx.de>
Cc: Igor Opaniuk <igor.opaniuk@foundries.io>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Ludwig Zenz <lzenz@dh-electronics.com>
Cc: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Cc: uboot-imx <uboot-imx@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>