arm64: zynqmp: Get rid of simple_itoa and replace it by snprintf

simple_itoa() is implemented only for !CONFIG_USE_TINY_PRINTF. Tiny printf
is normally used by SPL that's code which uses simple_itoa() has missing
reference. That's why refactor code by using on snprintf() instead of
strncpy()/strncat() combination. This change also descrease code size by
saving 24B based on buildman.

aarch64: (for 1/1 boards) all -22.0 rodata +2.0 text -24.0
         xilinx_zynqmp_virt: all -22 rodata +2 text -24
            u-boot: add: 0/0, grow: 0/-1 bytes: 0/-24 (-24)
              function                                   old     new   delta
              board_init                                 520     496     -24

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