lib: utils/fdt: Add helpers for generic driver initialization

Currently, each driver subsystem contains its own code for matching
drivers against the platform's devicetree blob. This bloats firmware
size because the several FDT scanning loops are almost exact copies of
each other, and is confusing because the loops do have some subtle
differences. Furthermore, the existing match algorithm is inefficient:
it scans the FDT structure separately for each driver in the list. A
faster algorithm scans the FDT blob only once, matching all drivers in
the list for each `compatible` property seen.

Add new helpers implementing this faster algorithm. Since they must
iterate through the list of drivers, the driver structure cannot be
opaque. However, since the driver list is an array of pointers, the
`struct fdt_driver` can be embedded in a subsystem-specific driver
structure if needed. These three helpers cover all existing use cases
for driver initialization within OpenSBI.

An additional benefit of centralized driver initialization is the
consistent use of fdt_node_is_enabled().

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

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