Adopted SK450 mixer for the gimbal control group

Added start of the gimbal driver for SK450

ROMFS/px4fmu_common/init.d/10019_sk450_deadcat

@@ -29,3 +29,5 @@ set MIXER sk450_deadcat
 
 set PWM_OUT 1234
 set PWM_MIN 1050
+
+gimbal start

ROMFS/px4fmu_common/mixers/sk450_deadcat.aux.mix

@@ -1,26 +1,32 @@
 Gimbal / payload mixer for all 6 aux outputs
 -----------------------------------------------------
 
+# gimbal roll
 M: 1
 O:      10000  10000      0 -10000  10000
-S: 0 0  10000  10000      0 -10000  10000
+S: 2 0  10000  10000      0 -10000  10000
 
+# gimbal pitch
 M: 1
 O:      10000  10000      0 -10000  10000
-S: 0 1  10000  10000      0 -10000  10000
+S: 2 1  10000  10000      0 -10000  10000
 
+# gimbal yaw
 M: 1
 O:      10000  10000      0 -10000  10000
-S: 0 2  10000  10000      0 -10000  10000
+S: 2 2  10000  10000      0 -10000  10000
 
+# gimbal shutter
 M: 1
 O:      10000  10000      0 -10000  10000
-S: 0 3  10000  10000      0 -10000  10000
+S: 2 3  10000  10000      0 -10000  10000
 
+# reserved and not used, yet
 M: 1
 O:      10000  10000      0 -10000  10000
-S: 1 0  10000  10000      0 -10000  10000
+S: 2 4  10000  10000      0 -10000  10000
 
+# parachute
 M: 1
 O:      10000  10000      0 -10000  10000
-S: 1 1  10000  10000      0 -10000  10000
+S: 2 7  10000  10000      0 -10000  10000