arm64: dts: ti: k3-j721e: Pulldown DQS on OSPI

PHY reads expect the DQS line to be pulled down before reads start. But
not all flashes drive DQS during the dummy cycle phase. Configure the
pinmux to pull it down to ensure the start of reads is detected
properly.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>