ARM: OC_ETM should not select ARM_AMBA

The CONFIG_ARM_AMBA symbol is selected by the platforms that support
AMBA, and the other platforms (e.g. orion) might not be able to
build that code at all when they do not support the clk API.

Instead, make sure that OC_ETM is only built on platforms that
do support the amba subsystem already.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>