Insert '_' to delimit clusters and register arrays

The 1170 chip family has a different CCM design than anything in the
10xx family. It has a CLOCK_ROOT cluster of size 79. Within that cluster
is an array of CLOCK_ROOT_SETPOINT registers, size 16.

imxrtral.py doesn't emit structs for clusters or arrays for register
arrays. Instead, it appends the cluster number and the register number
to the end of the symbol. For instance, the 13th setpoint register in
the first cluster is called CLOCK_ROOT_SETPOINT131.

However, the first setpoint register in the 31st cluster is also called
CLOCK_ROOT_SETPOINT131. This confuses the compiler (and humans); a build
with this CCM peripheral fails due to duplicate members in the register
block, duplicate modules, etc. This only happens when you have cluster
and register array dimensions that happen to overlap.

This commit changes the naming of registers derived from clusters. It
emits a '_' before the cluster number. In the example above, this means
that the symbols become CLOCK_ROOT_SETPOINT13_1 and
CLOCK_ROOT_SETPOINT1_31, which are unique and discernable. It's a
breaking change, and a workaround to explore 1170 support in the RAL.

imxrtral.py

@@ -1892,7 +1892,7 @@ def expand_cluster(node):
     nodes = []
     for register in node.findall('register'):
         addr = cluster_addr + get_int(register, 'addressOffset')
-        name = get_string(register, 'name') + str(cluster_idx)
+        name = get_string(register, 'name') + "_" + str(cluster_idx)
         clusr = copy.deepcopy(register)
         clusr.find('addressOffset').text = "0x{:08x}".format(addr)
         clusr.find('name').text = name