arm64: zynqmp: Use minimal size for environment variables

arm64: zynqmp: Use minimal size for environment variables

There is no reason to have huge space for variables because none is
using that. But there are some which are setup automatically.

baudrate=115200
fdtcontroladdr=ffffa0d0
stderr=dcc
stdin=dcc
stdout=dcc

Environment size: 72/124 bytes

Signed-off-by: Michal Simek <michal.simek@xilinx.com>