xilinx: Enable CMD_GREPENV/SETEXPR by default

Enable both of these commands in Xilinx SoCs to be able to use them in boot
scripts.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/1a612be7e09e9bc502f30e1f025441ccf8accba5.1661340513.git.michal.simek@amd.com