ARM: OMAP2+: Drop legacy platform data for am3 and am4 mailbox

ARM: OMAP2+: Drop legacy platform data for am3 and am4 mailbox

We can now probe devices with ti-sysc interconnect driver and dts
data. Let's drop the related platform data and custom ti,hwmods
dts property.

As we're just dropping data, and the early platform data init
is based on the custom ti,hwmods property, we want to drop both
the platform data and ti,hwmods property in a single patch.

Cc: Keerthy <j-keerthy@ti.com>
Cc: Suman Anna <s-anna@ti.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>