arm64: dts: ti: k3-am642-hummingboard-t: correct rs485 rts polarity

arm64: dts: ti: k3-am642-hummingboard-t: correct rs485 rts polarity

The RS485 transceiver RE (Receiver enable) and DE (Driver enable) are
shorted and connected to both RTS/CTS of the SoC UART.
RE is active-low, DE is active-high.

Remove the "rs485-rts-active-low" flag to match RTS polarity with DE,
and fix communication in both transmit and receive directions.

Fixes: d60483faf914 ("arm64: dts: add description for solidrun am642 som and evaluation board")
Signed-off-by: Josua Mayer <josua@solid-run.com>
Link: https://lore.kernel.org/r/20240504-ti-rs485-rts-v1-1-e88ef1c96f34@solid-run.com
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>