KEYS: trusted: Fix trusted key backends when building as module

Before this commit the kernel could end up with no trusted key sources
even though both of the currently supported backends (TPM and TEE) were
compiled as modules. This manifested in the trusted key type not being
registered at all.

When checking if a CONFIG_… preprocessor variable is defined we only
test for the builtin (=y) case and not the module (=m) case. By using
the IS_REACHABLE() macro we do test for both cases.

Fixes: 5d0682be

 ("KEYS: trusted: Add generic trusted keys framework")
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Andreas Rammhold <andreas@rammhold.de>
Tested-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>