Rename M2351 platform name to 'NU_PFM_M2351' from 'NUMAKER_PFM_M2351'

Originally its name is 'NUMAKER_PFM_M2351'. After adding suffix to distinguish
PSA/non-PSA targets, it will have pattern below which exceeds max 20 chars limited
by online database:

'NUMAKER_PFM_M2351'+'_'+['PSA', 'NOPSA']+'_'+['S', 'NS']

To fix the issue, the M2351 platform name is changed to 'NU_PFM_M2351'. Then it will
have pattern below for PSA/non-PSA targets, which fit in max 20 chars limit:

'NU_PFM_M2351'+'_'+['P', 'NP']+'_'+['S', 'NS']

Author: ccli8

src/mbed_os_tools/detect/platform_database.py

@@ -225,7 +225,7 @@
         u"1302": u"NUMAKER_PFM_NUC472",
         u"1303": u"NUMAKER_PFM_M453",
         u"1304": u"NUMAKER_PFM_M487",
-        u"1305": u"NUMAKER_PFM_M2351",
+        u"1305": u"NU_PFM_M2351",
         u"1306": u"NUMAKER_PFM_NANO130",
         u"1307": u"NUMAKER_PFM_NUC240",
         u"1308": u"NUMAKER_IOT_M487",