OMAP4: clock: Add 'virt_lcd_pclk' clock

Add virtual LCD pixel clock which is going to be used for DSS OPPs

Change-Id: Ia42668873d4dbe75deef22e97cec449a00f987d0
Signed-off-by: Vitaly Chernooky <vitaly.chernooky@ti.com>
Signed-off-by: Taras Kondratiuk <taras@ti.com>