Choice of UART for mach-mmp

[cjb]

The kernel code for MMP* is currently inflexible in that it demands the console UART be hardcoded for a given kernel image. But we want to boot the same kernel image on XO-1.75 and XO-4, and they use different UARTs. (The UART3 used by XO-1.75 is not used on XO-4.)

Tegra (another DT platform) uses a cookie in RAM to encode which UART should be used. We should have OFW do the same, probably with a "magic cookie,uart address" combination. If we find the cookie then we use that address, else we use UART3 to retain compatibility with old 1.75 firmware.

Until this happens, we can't make a single kernel image that provides serial console on XO-1.75 and XO-4, so it's important. Moreover, if we leave UART3 (for XO-1.75) as the default, it crashes XO-4 because the UART3 clock is not turned on.

Assigning first to Mitch to provide the cookie in OFW, then to kernel.

[dsd]

Lowering priority as we are not going for a single kernel for both platforms at this time.