OMAP3: serial: Check for zero-based physical addr

OMAP3: serial: Check for zero-based physical addr

This is for protecting a wrong mapping attempt of a zero-based
physical address.

The result is that, no serial port will be attempted to be mapped.

Also add an additional protection for NULL clocks before attempting
to enable them (if above condition applies)

Signed-off-by: Sergio Aguirre <saaguirre@ti.com>