Convert CONFIG_SYS_L2_PL310 to Kconfig

This converts CONFIG_SYS_L2_PL310 to Kconfig.

For omap2 and mvebu the 'select SYS_L2_PL310' locations were
determined using ./tools/moveconfig -i CONFIG_SYS_L2_PL310.

For mx6 I manually chose ARCH_MX6 as 'select' location. The
correctness has been verified using

	$ ./tools/moveconfig.py -f ARCH_MX6 ~SYS_L2_PL310 ~SYS_L2CACHE_OFF
	0 matches

That means whenever an ARCH_MX6 board had SYS_L2_PL310 disabled, this
was correctly reflected in SYS_L2CACHE_OFF. Thus it's safe to insert
the 'select' statement under ARCH_MX6.

Signed-off-by: Philip Oberfichtner <pro@denx.de>
diff --git a/configs/socfpga_sockit_defconfig b/configs/socfpga_sockit_defconfig
index 1e9dc14..0539d29 100644
--- a/configs/socfpga_sockit_defconfig
+++ b/configs/socfpga_sockit_defconfig
@@ -1,4 +1,5 @@
 CONFIG_ARM=y
+CONFIG_SYS_L2_PL310=y
 CONFIG_ARCH_SOCFPGA=y
 CONFIG_SYS_MALLOC_LEN=0x4000000
 CONFIG_ENV_SIZE=0x2000