MCP23XXX Refactor (#1560)

* Refactor MCP23XXX classes to consolidate shared code

* Update test mcp23xxx pin schemas

esphome/components/mcp23x08_base/__init__.py

@@ -0,0 +1,8 @@
+import esphome.codegen as cg
+from esphome.components import mcp23xxx_base
+
+AUTO_LOAD = ["mcp23xxx_base"]
+CODEOWNERS = ["@jesserockz"]
+
+mcp23x08_base_ns = cg.esphome_ns.namespace("mcp23x08_base")
+MCP23X08Base = mcp23x08_base_ns.class_("MCP23X08Base", mcp23xxx_base.MCP23XXXBase)