spi: Add SPI Flash controller

spi: Add SPI Flash controller

This adds an SPI flash controller which supports direct
memory-mapped access to the flash along with a manual
mode to send commands.

The direct mode can be set via generic to default to single
wire or quad mode. The controller supports normal, dual and quad
accesses with configurable commands, clock divider, dummy clocks
etc...

The SPI clock can be an even divider of sys_clk starting at 2
(so max 50Mhz with our typical Arty designs).

A flash offset is carried via generics to syscon to tell SW about
which portion of the flash is reserved for the FPGA bitfile. There
is currently no plumbing to make the CPU reset past that address (TBD).

Note: Operating at 50Mhz has proven unreliable without adding some
delay to the sampling of the input data. I'm working in improving
this, in the meantime, I'm leaving the default set at 25 Mhz.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>