ARM: opcodes: add opcodes definitions for ARM security extensions

The ARM security extensions introduced the smc instruction, which is not
supported by all versions of GAS.

This patch introduces opcodes-sec.h, so that smc is made available in a
similar manner to hvc.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Dave Martin <dave.martin@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>