board: ti: am64x: Use fdt functions for ram and bank init

Use the appropriate fdtdec_setup_mem_size_base and
fdtdec_setup_bank_size calls in dram_init and dram_bank_init to pull
these values from DT, where they are already available, instead of
hardcoding them.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>