drivers: crypto: rsa_mod_exp: Add manual relocation for ops->mod_exp()

This patch adds manual relocation for Modular Exponentiation if
CONFIG_NEEDS_MANUAL_RELOC is enabled.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>