omap: Allow omap_serial_early_init() for OMAP4430 board

This patch enables omap_serial_early_init() function for OMAP4430
SDP. Without this the bootup would throw oops in omap_serial_init().

Note that the ifndef CONFIG_ARCH_OMAP4 is split into two sections
to enable omap_serial_early_init(). This ifndef cannot be removed
until omap4 clock framework is implemented.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reviewed-By: Tony Lindgren <tony@atomide.com>
Reviewed-By: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>