doc: move i.MX7D/i.MX8MM A/B booting to board specific

Having "i.MX7D/i.MX8MM SRC_GPR10 PERSIST_SECONDARY_BOOT for bootloader A/B
switching" at the top level of the documentation tree does not make sense.
Move it to board specific information.

Fixes: 59e3d1bd4992 ("doc: imx: psb: Document usage of SRC_GPR10 PERSIST_SECONDARY_BOOT for A/B switching")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
5 files changed