arm: imx: cx9020: remove usage of mx53_dram_size

arm: imx: cx9020: 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().

However multiple calls are better than undefined behavior...
This fixes:
https://lists.denx.de/pipermail/u-boot/2017-November/313214.html
https://lists.denx.de/pipermail/u-boot/2017-December/314480.html

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