x86: Add function to get top of usable ram

x86: Add function to get top of usable ram

The memory layout calculations are done in calculate_relocation_address(),
and coreboot has its own version of this function. But in fact all we
really need is to set the top of usable RAM, and then the base version
will work as is.

So instead of allowing the whole calculate_relocation_address() function
to be replaced, create board_get_usable_ram_top() which can be used by
a board to specify the top of the area where U-Boot relocations to.

Signed-off-by: Simon Glass <sjg@chromium.org>