ipq806x: dts: ea8500: fix gpio based switch reset

ipq806x: dts: ea8500: fix gpio based switch reset

Linksys EA8500 uses designated GPIO to reset the switch and if the
switch isn't reset properly before first access, it can lead to unusable
switch after soft reboot of the device:

 libphy: GPIO Bitbanged MDIO: probed
 mdio_bus gpio-0: MDIO device at address 0 is missing.
 mdio_bus gpio-0: MDIO device at address 4 is missing.

Working case:

 libphy: GPIO Bitbanged MDIO: probed
 switch0: Atheros AR8337 rev. 2 switch registered on gpio-0

So this patch sets reset GPIO to active low for 15ms as datasheet says,
that chip reset is active low and active low duration must be greater
than 10ms.

Tested-by: Your Real Name <424778940z@gmail.com>
Reported-by: Your Real Name <424778940z@gmail.com>
Ref: https://github.com/openwrt/openwrt/pull/2047
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=2168
Signed-off-by: Petr Štetiar <ynezz@true.cz>