MIPS: Lantiq: Add helper to set PCI clock delay

MIPS: Lantiq: Add helper to set PCI clock delay

The PCI core has a register that allows us to set the nanosecond delay of the
PCI clock lane. This patch adds a helper function to allow setting this value.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4119/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>