kvargs: promote get from key as stable

kvargs: promote get from key as stable

The function rte_kvargs_get() is used by eal and pci bus driver since
its introduction in commit 3ab385063cb9 ("kvargs: add get by key") and
commit d2a66ad79480 ("bus: add device arguments name parsing"), in
dpdk 21.05.

Let's promote it as stable.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>