modules: trusted-firmware-m: fix incorrect check of <option> args

'IPC' and 'REGRESSION' are passed to the <option>
argument of cmake_parse_arguments, hence they are always defined,
and 'if (DEFINED)' would always return TRUE.

Use 'if' directly instead of 'if DEFINED' to check if these
options are set or not.

Ref: NCSDK-7702

Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>