arm64: dts: ti: k3-am64-evm/sk: Add OSPI flash DT node

Both AM64 EVM and SK have a 512Mb S28HS512T Octal SPI NOR flash.
Add DT node for the same.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20210318113757.21012-2-vigneshr@ti.com