board: developerbox: fix mem_map setup timing

The setup of global variable mem_map was moved into enable_caches()
by commit a70c75ca ("board: developerbox: move mem_map setup later")
since U-Boot was directly booted from NOR flash in XIP
and bss is not yet available in dram_init() at that time.
This has a problem, mem_map variable is used by
the get_page_table_size() to calculate the page table size,
but get_page_table_size() is called earlier than enable_caches()
which fills mem_map variable. With that, U-Boot fails to boot when
64GB DIMM is installed.

Currently U-Boot on the Developerbox board is not booted in XIP
and bss is available in dram_init(), let's move mem_map setup
in dram_init().

Signed-off-by: Masahisa Kojima <kojima.masahisa@socionext.com>