doc: imx: psb: Fix PERSIST_SECONDARY_BOOT bit location in GPR10

doc: imx: psb: Fix PERSIST_SECONDARY_BOOT bit location in GPR10

The PERSIST_SECONDARY_BOOT is in GPR10 address 0x30390098, adjust the
text which currently says it is in GPR0 while using the correct address
of GPR10.

Signed-off-by: Marek Vasut <marex@denx.de>
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: Marcel Ziswiler <marcel.ziswiler@toradex.com>