ARM: OMAP2+: Use omap initcalls

This way the initcalls don't run on other SoCs on multiplatform
kernels. Otherwise we'll get something like this when booting
on vexpress:

omap_hwmod: _ensure_mpu_hwmod_is_setup: MPU initiator hwmod mpu not yet registered
...
WARNING: at arch/arm/mach-omap2/pm.c:82 _init_omap_device+0x74/0x94()
_init_omap_device: could not find omap_hwmod for mpu
...
omap-dma-engine omap-dma-engine: OMAP DMA engine driver
...

Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
diff --git a/arch/arm/mach-omap2/serial.c b/arch/arm/mach-omap2/serial.c
index 04fdbc4..24e9ad3 100644
--- a/arch/arm/mach-omap2/serial.c
+++ b/arch/arm/mach-omap2/serial.c
@@ -254,7 +254,7 @@
 
 	return 0;
 }
-core_initcall(omap_serial_early_init);
+omap_core_initcall(omap_serial_early_init);
 
 /**
  * omap_serial_init_port() - initialize single serial port