configs/cubieboard2: bump U-Boot and kernel

configs/cubieboard2: bump U-Boot and kernel

Bump U-Boot to version 2021.10 and kernel to 5.15.11
Among other things fixes U-Boot build failure,
see https://gitlab.com/buildroot.org/buildroot/-/jobs/1915006081:
---------------------->8-------------------
  HOSTLD  scripts/dtc/dtc
/usr/bin/ld: scripts/dtc/dtc-parser.tab.o:(.bss+0x10): multiple definition of `yylloc'; scripts/dtc/dtc-lexer.lex.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status
---------------------->8-------------------

Signed-off-by: Alexey Brodkin <[email protected]>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <[email protected]>