ARM: zynq: Do not print message about boot device

This information is shown already that's why there is no reason to print it
again via custom prints.

U-Boot SPL 2020.01-03080-ga6214d033bd0 (Mar 05 2020 - 09:59:05 +0100)
mmc boot
Trying to boot from MMC1

or

U-Boot SPL 2020.01-03080-ga6214d033bd0 (Mar 05 2020 - 10:49:46 +0100)
qspi boot
Trying to boot from SPI

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