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>