ARM: dts: BCM5301X: correct RX delay and enable flow control on Asus RT-AC88U

The current 'rx-internal-delay-ps' property value on the Realtek switch
node, 2000, will be divided by 300, resulting in 6.66, which will be
rounded to the closest step value, 7. Change it to 2100 to be accurate.
See ef136837 ("net: dsa: rtl8365mb: set RGMII RX delay in steps of
0.3 ns") for reference.

Flow control needs to be enabled on both sides of the internal and
external switch. It is already enabled on the CPU port of the Realtek
switch so we also enable it on the external switch port of the Broadcom
switch as well.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>