arm64: zynqmp: Add mini mtest configuration

arm64: zynqmp: Add mini mtest configuration

This configuration is useful when you want to run small u-boot and
perform DDR memory test to make sure that DDR is properly configured.
It is use for board bringup because alternative u-boot memory tests is
quite good.
Configuration is running out of OCM.

As is done for others mini configurations 0x80 bytes for variables is
enough and only default variables are stored there.

Alternative memtest is enabled and also 2GB of DDR via DTS files.
Configuration is enabling ZYNQMP_PSU_INIT_ENABLED and include psu_init()
from zcu102 for testing purpose.
In case of size issue this can be moved to SPL configuration as is done
for mini_qspi configuration but it is not a problem now.

Log:
U-Boot 2018.11-00268-gbd58b8ba8915 (Nov 29 2018 - 15:33:35 +0100)

Model: ZynqMP MINI
Board: Xilinx ZynqMP
DRAM:  WARNING: Initializing TCM overwrites TCM content
2 GiB
EL Level:       EL3
In:    dcc
Out:   dcc
Err:   dcc
ZynqMP>

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