Move the clock-related properties in the DesignWare PCIe controller
bindings to 'optional' set of properties.

[bhelgaas: move to separate patch]
Signed-off-by: Bhupesh Sharma <bhupesh.sharma@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>