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>