[spibus] begin conversion of SPI transactions to spibus acquire/release semantics

[adrian]

Summary

This implements:

• spibus acquire/release semantics, so SPI bus transactions can be atomic
• sleep/wakeup in the bus acquire/release methods, so controllers don't have to implement their own serialisation (eg what bcm2835_spi does)
• convert all the spi devices I can find over to the new semantics
• convert some of the (simple!) spi controllers over to the new semantics

However, it doesn't yet implement all controllers, as it's not entirely
clear how to hold CS asserted for the whole transaction set.

It's quite possible that a follow up commit will be to let the controller
specify whether CS can be held across transactions or not, so potential
consumers (eg mmcspi) can decide whether the work or not.

Test Plan

• Testing on AR9331 SoC, ned to test on others

[adrian]

Make the exynos5_ec spi access mirror the same behaviour as before.

[mmel]

I'm not sure if we can handle CS in acquire/release bus. With MMCSPI, for example, we must be able to issue some clock pulses with CS de-asserted.

[adrian]

interesting. pkelsey and loos apparently had mmcspi going with CS asserted during bus acquire/release, right?

I'm okay committing the acquire/release without moving the CS assertion bits in?