Add AMBA bus support to platform devices

AMBA specification from Arm is a freely available open
standard for the connection and management of functional
blocks in a system-on-chip.
AMBA devices create a device kernel object in /sys/devices/platform
and a kernel object of the bus for the respective device in
/sys/bus/amba. For e.g. for pl180 mmc:
device: /sys/devices/platform/1c050000.mmci
bus: /sys/bus/amba/devices/1c050000.mmci

In first stage init, android creates a symlink from /sys/devices/platform/...
to /dev/block/platform/... for all partitions and then to
/dev/block/by-name for the boot_device used. However this is only done
for devices that "attach" to bus/platform. This patch adds support for
platform devices that attach to bus/amba.

Signed-off-by: Usama Arif <usama.arif@arm.com>
Change-Id: I9da7456407c4f62335c959502d8b2d16026fddfc