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

arch/arm/cpu/armv7/sunxi/dram.c

@@ -262,6 +262,10 @@ static void mctl_setup_dram_clock(u32 clk, u32 mbus_clk)
 	reg_val &= ~CCM_PLL5_CTRL_K_MASK;		/* set K to 0 (x1) */
 	reg_val &= ~CCM_PLL5_CTRL_N_MASK;		/* set N to 0 (x0) */
 	reg_val &= ~CCM_PLL5_CTRL_P_MASK;		/* set P to 0 (x1) */
+#ifdef CONFIG_OLD_SUNXI_KERNEL_COMPAT
+	/* Old kernels are hardcoded to P=1 (divide by 2) */
+	reg_val |= CCM_PLL5_CTRL_P(1);
+#endif
 	if (clk >= 540 && clk < 552) {
 		/* dram = 540MHz */
 		reg_val |= CCM_PLL5_CTRL_M(CCM_PLL5_CTRL_M_X(2));

board/sunxi/Kconfig

@@ -20,6 +20,13 @@ config SYS_SOC
 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 ""