Add SSP SPI driver for Intel Baytrail SoC

[gonzo]

Summary

Add SPI driver for multi-functional serial port (SSP) that can
be found in Intel's SoCs. I only have Baytrail SoC so this is the only
supported hardware for now.

Test Plan

Use SSD1306 for basic tests

[br]

looks good

sys/dev/intel/spi.c

79 ↗

(On Diff #23229)

we usually do
​#define<tab>INTELSPI_SSPREG_SSCR0
​#define<tab><space>SSCR0_SCR(n)
instead of
​#define<tab>INTELSPI_SSPREG_SSCR0
​#define<tab><tab>SSCR0_SCR(n)

156 ↗

(On Diff #23229)

        if (sssr & SSSR_TNF)
	​                return (0);

        return (1);

385 ↗

(On Diff #23229)

remove empty line here

[manu]

Wouldn't be better to directly add support for mode/clock setting in transfer ?

[gonzo]

In D8896#184336, @manu wrote:

Wouldn't be better to directly add support for mode/clock setting in transfer ?

Yes. I'll probably do this before committing the driver

[gonzo]

In D8896#184406, @gonzo wrote:

In D8896#184336, @manu wrote:

Wouldn't be better to directly add support for mode/clock setting in transfer ?

Yes. I'll probably do this before committing the driver

I will add mode/clock support in the next round of change.