We tried using Zynq UltraScale+ MPSoC Crypto Hardware with block size of 4 bytes, It is taking a lot of time for encrypting and decrypting the payload of size > 20MB.
Question 1. Can we increase block size of AES GCM Driver in runtime or patching it is the only way? - Link to Xilinx Page
Question 2. There is Arm v8 crypto extension in the board but where can we find the driver for this for petalinux for Xilinx MPSoc.
Take a look at WP512 to see if there are info in there. For the driver itself try the Embedded OS forum.