mips: octeon: Report full DDR size in dram_init() to gd->ram_size

With this patch, gd->ram_size now holds to full RAM size detected by the
DDR init code. It introduces the get_effective_memsize() function to
report the maximum usable RAM size in U-Boot to the system instead.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Aaron Williams <awilliams@marvell.com>
Cc: Chandrakala Chavva <cchavva@marvell.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>