board: ti: am64x: Init DRAM size in R5/A53 SPL

Call dram_init_banksize() from spl_board_init() otherwise TFTP download
fails due to lmb_get_free_size() not able to find unreserved region due
to lack of DRAM size info. Required to support Ethernet boot on AM64x.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>