ARM: uniphier: Move uniphier_mem_map_init() call into dram_init()

The function uniphier_mem_map_init() is to change global variable
'mem_map', which is referenced to get_page_table_size() to calculate
the size of page table.

However, uniphier_mem_map_init() is called after get_page_table_size(),
so the size of page table and 'mem_map' become inconsist each other.
After all, U-Boot fails to boot on chip with memory map different from
default map,

uniphier_mem_map_init() should be moved to dram_init(), which is
called before get_page_table_size().

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>