ARM: OMAP2+: Fix compile if CONFIG_HAVE_ARM_SMCCC is not set

Recent omap changes added runtime checks to use omap_smccc_smc()
when optee is configured in dts. As the omap-secure code can be
built for ARMv6 only without ARMv7 and use custom smc calls, we
now get a build error:

omap-secure.c:(.text+0x94): undefined reference to `__arm_smccc_smc'

As there secure calls are not used for ARMv6, we should not build
secure-common, and not call omap_secure_init() for omap2.

Fixes: c37baa06 ("ARM: OMAP2+: Fix undefined reference to omap_secure_init")
Reported-by: kbuild test robot <lkp@intel.com>
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Andrew F. Davis <afd@ti.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Steven Price <steven.price@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>