common: add an option to skip DM pre-relocation

common: add an option to skip DM pre-relocation

For some platforms like Qualcomm, it isn't necessary to perform a full
DM init and scan prior to relocation, it's also particularly slow since
it runs with dcache disabled and prior to building the livetree.

The only device which needs to be probed pre-reloc is the serial
port (otherwise U-Boot will panic), however this can be found through
/chosen/stdout-path.

Therefore we can avoid scanning the entire FDT and binding devices,
instead just binding the serial port and clock driver on-demand.

This decreases the total time from power on to reaching the interactive
U-Boot shell be about 50% (from ~2.8s to 1.8s).

Signed-off-by: Casey Connolly <casey.connolly@linaro.org>