firmware: trusted_foundations: add ARMv7 dependency

The "+sec" extension is invalid for older ARM architectures, but
the code can now be built on any ARM configuration:

/tmp/trusted_foundations-2d0882.s: Assembler messages:
/tmp/trusted_foundations-2d0882.s:194: Error: architectural extension `sec' is not allowed for the current base architecture
/tmp/trusted_foundations-2d0882.s:201: Error: selected processor does not support `smc #0' in ARM mode
/tmp/trusted_foundations-2d0882.s:213: Error: architectural extension `sec' is not allowed for the current base architecture
/tmp/trusted_foundations-2d0882.s:220: Error: selected processor does not support `smc #0' in ARM mode

Add a dependency on ARMv7 for the build.

Fixes: 4cb5d9ec

 ("firmware: Move Trusted Foundations support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>