staging: mt7621-pci: add some definitions for enabling and disabling GEN and GEN1 clocks

Instead of use hardcoded values when calling 'rt_sysc_m32' for enabling
and disabling RALINK_PCIE_CLK_GEN and RALINK_PCIE_CLK_GEN1 create
some preprocessor definitions.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>