ARM: omap2: fix am43xx build without L2X0

When CONFIG_CACHE_L2X0 is disabled, the am43xx specific suspend
implemnentation fails to link:

arch/arm/mach-omap2/sleep43xx.o: In function `get_l2cache_base':
(.text+0x180): undefined reference to `omap4_get_l2cache_base'

This adds an #ifdef protection around the code, like we do for am44xx.

Fixes: 41d37e61 ("ARM: OMAP2+: Introduce low-level suspend code for AM43XX")
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>