[dev][platform-bus] separate regular metadata from bootloader metadata

In the platform bus protocol, we use the pbus_metadata_t struct
for board drivers to pass metadata to the platform devices.
Instead of overloading this struct to work both for immediately
provided metadata and metadata passed from the bootloader, we now have
a separate struct pbus_boot_metadata_t used for specifying bootloader
metadata to be assigned to a platform device.

ZX-2513 #in progress

TEST: booted VIM2, astro and gauss and verified that drivers that
use metadata started up successfully.

Change-Id: I7065fd497646f386987cb5dc317d1c66daa8df0a

[platform-bus] Remove proxying of USB mode switch protocol

This protocol is only being used for Hikey960, which needs to change some GPIOs
when switching USB mode. But now that we have support for child platform devices,
this logic can be pushed out of the board driver and into a platform device driver
which has the dwc3 driver as a child.

TEST: manual testing on hikey960 and imx8m-evk
Change-Id: Ia3c1b9540d0813d3d873f22007e3e22efb219975