all: rename __reg to __volatile

It is less magical now: it only marks loads and stores as volatile. So
the name '__reg' is a bit wrong now.

gen-device-svd.py

@@ -158,10 +158,10 @@ package {pkgName}
 import "unsafe"
 
 // Magic type name for the compiler.
-type __reg uint32
+type __volatile uint32
 
 // Export this magic type name.
-type RegValue = __reg
+type RegValue = __volatile
 
 // Some information about this device.
 const (
@@ -193,14 +193,14 @@ const (
             if address < register['address']:
                 numSkip = (register['address'] - address) // 4
                 if numSkip == 1:
-                    out.write('\t_padding{padNumber} __reg\n'.format(padNumber=padNumber))
+                    out.write('\t_padding{padNumber} __volatile\n'.format(padNumber=padNumber))
                 else:
-                    out.write('\t_padding{padNumber} [{num}]__reg\n'.format(padNumber=padNumber, num=numSkip))
+                    out.write('\t_padding{padNumber} [{num}]__volatile\n'.format(padNumber=padNumber, num=numSkip))
                 padNumber += 1
 
-            regType = '__reg'
+            regType = '__volatile'
             if register['array'] is not None:
-                regType = '[{}]__reg'.format(register['array'])
+                regType = '[{}]__volatile'.format(register['array'])
             out.write('\t{name} {regType}\n'.format(**register, regType=regType))
 
             # next address