airoha: rework RAM size handling to support multiple RAM size

airoha: rework RAM size handling to support multiple RAM size

There are multiple version of the same reference board with different
RAM size and it's not enough to base the RAM size entirely from DT. To
better support it use the get_ram_size way to scan for the actual RAM
size of Airoha SoC and increase the size of the memory map.

Also rework the memory map to account for 2 memory map. The first one of
2GB for 32bit DMA and for safe usage of U-Boot. The second one for the
rest of the RAM since up to 8GB are supported.

Reviewed-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>