arm/airoha: an7581 ignores CFG_MAX_MEM_MAPPED value

arm/airoha: an7581 ignores CFG_MAX_MEM_MAPPED value

This partly fix commit 726404a66c773 ("airoha: rework RAM size handling
to support multiple RAM size")

The function get_effective_memsize() do not see non-global defines
of CFG_MAX_MEM_MAPPED, so the effective memory size will not be changed.

Fix the issue by putting definition of CFG_MAX_MEM_MAPPED to the proper
place.

Fixes: 726404a66c773 ("airoha: rework RAM size handling to support multiple RAM size")
Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>