sunxi: Add CONFIG_OLD_SUNXI_KERNEL_COMPAT Kconfig option

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
diff --git a/board/sunxi/Kconfig b/board/sunxi/Kconfig
index 449c046..31a1503 100644
--- a/board/sunxi/Kconfig
+++ b/board/sunxi/Kconfig
@@ -20,6 +20,13 @@
 config FDTFILE
 	string "Default fdtfile env setting for this board"
 
+config OLD_SUNXI_KERNEL_COMPAT
+	boolean "Enable workarounds for booting old kernels"
+	default n
+	---help---
+	Set this to enable various workarounds for old kernels, this results in
+	sub-optimal settings for newer kernels, only enable if needed.
+
 config MMC0_CD_PIN
 	string "Card detect pin for mmc0"
 	default ""