arm64: zynqmp: avoid out of buffer access

strncat(a, b, c) appends a maximum of c characters plus the 0 byte
to a.

In board_init we first write 4 characters plus 0 byte to version.
So only ZYNQMP_VERSION_SIZE - 5 additional characters fit into
version.

The problem was indicated by cppcheck.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>