ARM: omap2: fix am43xx dependency on l2x0 cache

Commit d941f86f ("ARM: l2c: AM43x: add L2 cache support") enabled
the L2 cache support for the am43xx SoC, but caused a build regression
when the driver for that cache controller is disabled:

arch/arm/mach-omap2/built-in.o: In function `am43xx_init_early':
:(.init.text+0xb20): undefined reference to `omap_l2_cache_init'

This did not happen for OMAP4, which has the same call, but enables
the l2x0 driver unconditionally. We could do the same thing for
am43xx, but it seems better to allow turning it off and make the
code work in either case.

This adds an inline wrapper for omap_l2_cache_init for the disabled
case, and removes the 'select' from OMAP4 so it becomes a user
visible option.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Cc: linux-omap@vger.kernel.org