char: xdevcfg: Set PCAP_RATE_EN for encrypted mode

PCAP_RATE_EN must be set when the AES engine is being used to decrypt
configuration data for either the PS or PL.

Signed-off-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>