ARM: dts: am335x-nano: Enable RS485 mode for UART3 & 4

UART3 & 4 are both RS485 ports.
So we need to configure and enable this by default.

Signed-off-by: Mark Jackson <mpfj@newflow.co.uk>
Message-Id: <20221130140547.295859-3-mpfj@newflow.co.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>