arm64: dts: ti: k3-am64-main: Add OSPI node

AM64 SoC has a single Octal SPI (OSPI) instance under Flash SubSystem
(FSS).  Add DT entry 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/20210318113757.21012-1-vigneshr@ti.com