mcu-gateway: migrate modbus_controller off deprecated register_count

register_count is derived from value_type now; drop the now-redundant
register_count: 1 on the temp/humidity sensors, and replace the
Solar Pressure sensor's register_count: 2 (its device always returns
2 registers per read) with a hidden padding sensor on the next
register using reuse_previous_range: true, per the new
modbus_controller range-grouping mechanism.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

mcu-gateway.yaml

@@ -129,7 +129,6 @@ sensor:
     register_type: read
     value_type: U_WORD
     accuracy_decimals: 1
-    register_count: 1
     filters:
       - multiply: 0.1
   - platform: modbus_controller
@@ -152,7 +151,6 @@ sensor:
     register_type: read
     value_type: U_WORD
     accuracy_decimals: 1
-    register_count: 1
     filters:
       - multiply: 0.1
   - platform: modbus_controller
@@ -192,11 +190,18 @@ sensor:
     register_type: holding
     value_type: U_WORD
     accuracy_decimals: 1
-    register_count: 2
     filters:
       - calibrate_linear:
           - 50 -> 0
           - 450 -> 5
+  - platform: modbus_controller
+    modbus_controller_id: modbus_adc_1
+    id: adc_1_1_padding
+    internal: true
+    address: 1
+    register_type: holding
+    value_type: U_WORD
+    reuse_previous_range: true
 
 switch:
   - platform: modbus_controller