Add a Kconfig option which users can select when they want to boot older
kernels, e.g. the linux-sunxi 3.4 kernels. For now this just forces the pll5
"p" value to 1 (divide by 2) as that is what those kernels are hardcoded too,
in the future this may enable further workarounds.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Tom Rini <trini@ti.com>
--
Changes in v2:
-s/CONFIG_OLD_KERNEL_COMPAT/CONFIG_OLD_SUNXI_KERNEL_COMPAT.
-Move the code block setting P(1) for old kernels to where P gets cleared