omap2+: Fix omap_serial_early_init to work with init_early hook

The new init_early hook happens at the end of setup_arch,
which is too early for kzalloc. However, there's no need
to call omap_serial_early_init that early, so fix this
by setting it up as a core_initcall.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Tested-by: Kevin Hilman <khilman@ti.com>