mvebu: add 5.10 as a testing kernel

mvebu: add 5.10 as a testing kernel

Keep 5.4 as stable until further validation.

Tested on Turris Omnia (Rui Salvaterra) and ESPRESSObin v5 (Tomasz
Maciej Nowak). Cortex-A{53,72} subtargets are only build-tested.

Reviewed-by: Tomasz Maciej Nowak <tmn505@gmail.com>
Tested-by: Tomasz Maciej Nowak <tmn505@gmail.com>
Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
[added comment about tests]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>