SPM: Invert tfm_spm_check_buffer_access() logic

Invert the logic for the tfm_spm_check_buffer_access() function
to conform to "0 means success" and align to other APIs. Refactor
the function itself to provide different return codes on error and
a default failure return instead of default success.

Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
Change-Id: I40ef814a472375cdb2c40ac75dd5f605a9eccbfe