omap4: Fix UART4 platform data on omap4

This patch removes the unnecessary UART4 platform which is under
data is wrong because of this

There is a separate platform structure for UART4

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