README.mpc85xx-sd-spi-boot: Suggest the NXP boot format github repo

As explained in the text at the bottom of the page
https://source.codeaurora.org/external/qoriq/qoriq-yocto-sdk/boot-format

:

"QUIC repositories on this site will not receive any updates after
March 31, 2022, and will be deleted on March 31, 2023."

Point to the NXP boot format github repo instead.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Pali Rohár <pali@kernel.org>