clk: sifive: Add pcie_aux clock in prci driver for PCIe driver

We add pcie_aux clock in this patch so that pcie driver can use
clk_prepare_enable() and clk_disable_unprepare() to enable and disable
pcie_aux clock.

Link: https://lore.kernel.org/r/20210504105940.100004-2-greentime.hu@sifive.com


Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>