The CAAM in IMX parts doesn't support public key hardware acceleration
(PKHA), so don't use RSA_FREESCALE_EXP. If you try to use it on IMX
(assuming you have the clocks enabled first) you will get back an
"Invalid KEY Command" error since PKHA isn't a valid key destination for
these parts.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>