arm64: dts: ti: k3-am64: Enable GPMC and ELM nodes at the board level

The GPMC node defined in the top-level AM64x SoC dtsi files are incomplete
and will not be functional unless it is extended with pinmux information.

As the pinmux is only known at the board integration level, this node
should only be enabled when provided with this information.

Disable the GPMC node in the dtsi file. Since the ELM is made to work
with the GPMC, disable it too.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Bryan Brattlof <bb@ti.com>
Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20221017192532.23825-11-afd@ti.com