x86: tangier: Find proper memory region for relocation

It appears that U-Boot works by luck on Intel Edison board because the amount
of RAM is less than 1 GB and standard way of calculating the top of it work
for this configuration. However, this won't work if the amount of RAM is
different and split differently in address space. We have to find the suitable
window correctly.

Find proper memory region for relocation by scanning MMAP SFI table in
board_get_usable_ram_top() callback.

According to the address map documentation the Main Memory is guaranteed to lie
in the 0..2 GB range, that's why we limit search by this range.

Fixes: e71de54a ("x86: Add Intel Tangier support")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: fixed a typo in the commit message]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>