ath79-tiny: enable 4k sectors

ath79-tiny: enable 4k sectors

This enables using 4kiB sectors as erase blocks for 4MiB NOR flash ICs
that support it.

Writeable jffs2 overlay used to store settings requires a partition with
at least 5 erase blocks, so using small sectors is essential for devices
with 4MiB flash.

Sysupgrading a device running firmware without this feature will likely
not allow to preserve configs automatically but since ath79 is
considered to be in a "technology preview" state it shouldn't be a
problem.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>