ath79: cleanup mikrotik routerboot partitions

ath79: cleanup mikrotik routerboot partitions

For some reason useless labels and aliases have been propagated through
copy-paste. Before the issue spreads any further, this patch cleans up
all relevant DTS files to the canonical form, bringing ath79 in line
with other mikrotik platforms (ramips and ipq40xx).

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>