eal: promote wait until equal API as stable

eal: promote wait until equal API as stable

rte_wait_until_equal_xx APIs were introduced in 19.11 release
and there were no changes in the public APIs since then, it
should be mature enough to remove the experimental tag.

Signed-off-by: Joyce Kong <joyce.kong@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: David Christensen <drc@linux.vnet.ibm.com>