OMAP: WDT: Split OMAP1 and OMAP2PLUS device registration

This patch splits omap_init_wdt() into separate omap_init_wdt()
functions under mach-omap1 and mach-omap2 and set them up with
subsys_initcall.

Also it uses  omap_device_build() API instead of
platform_device_register() for watchdog timer device registration
for OMAP2plus chips.

For OMAP2plus chips, the device specific data defined in centralized
hwmod database will be used.

Signed-off-by: Charulatha V <charu@ti.com>
Acked-by: Cousson, Benoit <b-cousson@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>