da8xx: fixup ARM relocation support

Split the existing dram_init for da8xx when ARM reloc is enabled, like the
changes to arch/arm/cpu/arm926ejs/orion5x/dram.c in
0f234d263b17ccf1b8fd776eb8c15b7cdb27a887 by Heiko Schocher <hs@denx.de>.

Without these changes gd->ram_size is '0' which leads to incorrect relocation
when CONFIG_SYS_ARM_WITHOUT_RELOC is defined and the board does not boot.

We use get_ram_size to dynamically calculate the available RAM because it runs
on different board version with different ram, as suggested by Heiko in private
communication.

Tested on a da850evm with 128M of DDR2 installed; with both
CONFIG_SYS_ARM_WITHOUT_RELOC defined and undefined.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Reviewed-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
CC: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
CC: Heiko Schocher <hs@denx.de>