powerpc/xics: Use of_address_count()

icp_native_init_one_node() only needs the number of entries in "reg".
Replace the open coded "reg" parsing with of_address_count() to get the
number of "reg" entries.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230327223056.820086-1-robh@kernel.org