memory: omap-gpmc: Add support for GPMC on AM64 SoC

The TI's AM64 SoC has the GPMC module. Add compatible for it.

Traditionally GPMC external addresses have always been mapped to first
1GB physical address. However newer platforms, can have it mapped
at different locations. Support this address provision via device tree.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Link: https://lore.kernel.org/r/20211221131757.2030-3-rogerq@kernel.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>