char: devcfg: Fix AES decryption error for non zero key

Keep PROG_B active for 5ms to allow the eFuse key to be read from the
array and cfg_reset to trigger the key expansion after the key has
loaded.

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