serial: Support an early UART for debugging

This came up in a discussion on the mailing list here:

https://patchwork.ozlabs.org/patch/384613/



My concerns at the time were:
- it doesn't need to be written in assembler
- it doesn't need to be ARM-specific

This patch provides a possible alternative. It works by allowing any serial
driver to export one init function and provide a putc() function. These
can be used to output debug data before the real serial driver is available.

This implementation does not depend on driver model, and it is possible for
it to operate without a stack on some architectures (e.g. PowerPC, ARM). It
provides the same features as the ARM-specific debug.S but with more UART
and architecture support.

Signed-off-by: Simon Glass <sjg@chromium.org>