Add support for most recent memory chips on BPi M5 / M2PRO

[igorpecovnik]

Description

• move FIP signing code from meson-g12b.conf's uboot_custom_postprocess() to the board's own post_uboot_custom_postprocess hook
• fetch a FIP blob repo from vendor developers fork as it wasn't merged upstream yet

Jira reference number AR-1700

How Has This Been Tested?

• Build test as HW is n/a

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules