ARM: davinci: dm644x: remove unnecessary postcore_initcall()

dm644x_init_devices() is unnecessarily implemented as a
postcore_initcall().

This does not cause any issues now, but creates issues when
converting to common clock framework where clocks themselves
become platform devices and are initialized later. Because of
this, device clock acquisition and runtime PM on that device
fails.

Move the call to .init_machine() callback along with rest of
platform device's registration.

While at it, convert it to return void since return value is
ignored by caller anyway.

Reviewed-by: David Lechner <david@lechnology.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>