crypto: mxs-dcp - Only set OTP_KEY bit for OTP key

While MXS_DCP_CONTROL0_OTP_KEY is set, the CRYPTO_KEY (DCP_PAES_KEY_OTP)
is used even if the UNIQUE_KEY (DCP_PAES_KEY_UNIQUE) is selected. This
is not clearly documented, but this implementation is consistent with
NXP's downstream kernel fork and optee_os.

Signed-off-by: Sven Schwermer <sven@svenschwermer.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>