ARM: cache: Allow SPL to build cache-pl310.c

Introduce the new Kconfig symbol CONFIG_SPL_SYS_L2_PL310 to allow the
SPL to build cache-pl310.c.

Before this commit, the SPL could enable the PL310 L2 cache [1], but the
cache maintenance functions from cache-pl310.c were only useable for
non-SPL builds.

After enabling the cache one must be able to flush it, too. Thus this
commit allows cache-pl310.c to be included in the SPL build.

[1] See for example arch/arm/mach-imx/cache.c: v7_outer_cache_enable()

Signed-off-by: Philip Oberfichtner <pro@denx.de>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 456a7fa..d6c5658 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -493,6 +493,11 @@
 	help
 	  Enable support for ARM PL310 L2 cache controller in U-Boot
 
+config SPL_SYS_L2_PL310
+	bool "ARM PL310 L2 cache controller in SPL"
+	help
+	  Enable support for ARM PL310 L2 cache controller in SPL
+
 config SYS_L2CACHE_OFF
 	bool "L2cache off"
 	help