arm64: dts: ti: k3-am64-main: Add GPMC memory controller node

The GPMC is a unified memory controller dedicated for interfacing
with external memory devices like
  - Asynchronous SRAM-like memories and ASICs
  - Asynchronous, synchronous, and page mode burst NOR flash
  - NAND flash
  - Pseudo-SRAM devices

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Bryan Brattlof <bb@ti.com>
Link: https://lore.kernel.org/r/20220802104456.11069-2-rogerq@kernel.org