of/fdt: add FDT serial scanning for earlycon

This adds FDT parsing of {linux,}stdout-path to setup an early serial
console. Enabling of the early console is triggered with "earlycon"
(with no options) on the kernel command line.

Platforms must either have fixmap permanent mapping support,
have a functioning ioremap when early params are parsed, or explicitly
call early_init_dt_scan_chosen_serial from architecture code.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Grant Likely <grant.likely@linaro.org>