x86: Adjust search range for sysinfo table

Avoid searching starting at 0 since this memory may not be available,
e.g. if protection against NULL-pointer access is enabled. The table
cannot be there anyway, since the first 1KB of memory was originally
used for the interrupt table and coreboot avoids it.

Start at 0x400 instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>