mk: fix quoting for ARM mtune argument

mk: fix quoting for ARM mtune argument

The mtune argument needs an unquoted argument, otherwise the compiler
complains:

  arm-buildroot-linux-gnueabi-gcc.br_real: error:
    unrecognized argument in option ‘-mtune="cortex-a9"’

Fixes: 02a8686263de ("mk: introduce ARMv7 architecture")
Fixes: 4a7e4626975a ("mk: introduce NXP dpaa2 architecture based on armv8-a")
Cc: stable@dpdk.org
Reported-by: Jan Viktorin <viktorin@rehivetech.com>
Signed-off-by: Pascal Mazon <pascal.mazon@6wind.com>
Tested-by: Jan Viktorin <viktorin@rehivetech.com>