x86: spi: Rewrite logic for obtaining the SPI memory map

At present this logic does not work on link and samus, since their SPI
controller is not a PCI device, but a child of the PCH.

Unfortunately, fixing this involves a lot of extra logic. Still, this was
requested in the review of the fix-up patch, so here it is.

Fixes: 92842147 ("spi: ich: Add support for get_mmap() method")
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com> (on Intel minnowmax)