arm: zynq: Remove checkboard and enable DISPLAY_CPUINFO

Now that showing silicon version is part of the CPU
info display, let's remove checkboard().

Note that the generic show_board_info() will still
show the DT 'model' property. For instance:

U-Boot 2018.05-rc2-00025-g611b3ee0159b (Apr 19 2018 - 11:23:12 +0200)

CPU:   Zynq 7z045
Silicon: v1.0
Model: Zynq ZC706 Development Board
I2C:   ready

Based on patches from Ariel D'Alessandro <ariel@vanguardiasur.com.ar>,
and Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>

mini configuration doesn't need to show this information.

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