arm: imx: mx53loco: remove usage of mx53_dram_size

Static variables are not available during board_init_f().
'static uint32_t mx53_dram_size[2];' was used in board specific
dram_init(), dram_init_banksize() and get_effective_memsize() to avoid
multiple calls to get_ram_size().

Reused dram initialization functions from arch/arm/mach-imx/mx5/mx53_dram.c

Signed-off-by: Patrick Bruenn <p.bruenn@beckhoff.com>