libpayload: usb: Allow direct instantiation of MMIO host controllers

The existing USB_MEMORY mechanism to instantiate non-PCI host
controllers is clunky and inflexible... most importantly, it doesn't
allow multiple host controllers of the same kind. This patch replaces it
with a function that allows payloads to directly instantiate as many
host controllers of whatever type they need.

Change-Id: Ic21d2016a4ef92c67fa420bdc0f0d8a6508b69e5
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/169454
Reviewed-by: Stefan Reinauer <reinauer@google.com>
(cherry picked from commit b6e95c39dd91f654f0a345f17b3196f56adf4891)
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/6644
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>