x86: spi: Add helper functions for Intel Fast SPI

x86: spi: Add helper functions for Intel Fast SPI

Most x86 CPUs use a mechanism where the SPI flash is mapped into the very
top of 32-bit address space, so that it can be executed in place and read
simply by copying from memory. For an 8MB ROM the mapping starts at
0xff800000.

However some recent Intel CPUs do not use a simple 1:1 memory map. Instead
the map starts at a different address and not all of the SPI flash is
accessible through the map. This 'Fast SPI' feature requires that U-Boot
check the location of the map. It is also possible (optionally) to read
from the SPI flash using a driver.

Add support for booting from Fast SPI. The memory-mapped version is used
by both TPL and SPL on Apollo Lake.

In respect of a SPI flash driver, the actual SPI driver is ich.c - this
just adds a few helper functions and definitions.

This is used by Apollo Lake.

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