ddr: Rename CONFIG_DDR_FIXED_SIZE to CONFIG_SYS_SDRAM_SIZE

We have a number of CONFIG symbols to express the fixed size of system
memory.  For now, rename CONFIG_DDR_FIXED_SIZE to CONFIG_SYS_SDRAM_SIZE
and adjust usage to match that CONFIG_SYS_SDRAM_SIZE expects the entire
size rather than MiB.

Signed-off-by: Tom Rini <trini@konsulko.com>