arm64: dts: ti: k3-am642-sk: Add OSPI DT node

AM642 SK has S28HS512T flash connected OSPI controller. Add DT entries
for the same.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>