Add the MXN currency definition

MXP is obsolete as even listed in the currency list. The currency of Mexico is as for the last almost 2 decades MXN and should be as follows:

     'MXN' => array(
       'code' => 'MXN',
       'symbol' => '$',
       'name' => t('Mexican New Peso'),
       'numeric_code' => '484',
       'symbol_placement' => 'before',
       'code_placement' => '',
       'minor_unit' => t('Centavo'),
       'major_unit' => t('Peso'),
     ),