mailbox: arm_mhu: re-factor data structure to add doorbell support

In order to support doorbells, we need a bit of reword around data
structures that are per-channel. Since the number of doorbells are
not fixed though restricted to maximum of 20, the channel assignment
and initialization is move to xlate function.

This patch also adds the platform data for the existing support of one
channel per physical channel.

Change-Id: I8d699c7dcca2a6e272a39582b06a29c14a7e3a62
Cc: Alexey Klimov <alexey.klimov@arm.com>
Cc: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>