ARM: orion5x: use mac_pton() helper

Instead of custom approach let's use generic helper function.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Detlef Vollmann <dv@vollmann.ch>
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Tested-by: Detlef Vollmann <dv@vollmann.ch> #on DNS-323
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>