Use _common naming scheme for MKS Robin E3 & SKR Mini E3 boards

[thisiskeithb]

Description

Use *_common.h naming scheme for MKS Robin E3 & MKS Mini E3 boards.

Benefits

FIle names will be in line with other boards that share a *_common.h pins file and more importantly, de-dupe pins for the MKS Robin E3 (w/ integrated drivers) & E3D (w/ removable drivers) for easier editing since they share most of the same pin definitions.

Related Issues

None.

[thisiskeithb]

The Robin E3D *_CS_PIN pins could have been moved to the common pins using a #define MKS_ROBIN_E3D/#ifdef MKS_ROBIN_E3D trick like we did with the SKR Mini E3 V2 & common pins, but I wasn't sure if that would be too much clutter.

If it makes sense to group them with the other stepper pins, I'm happy to move them over to the common file and add a check. Something like:

//
// Steppers
//
#define X_STEP_PIN                          PC0
#define X_DIR_PIN                           PB2
#define X_ENABLE_PIN                        PC13
#ifdef MKS_ROBIN_E3D
  #ifndef X_CS_PIN
    #define X_CS_PIN                        PC7
  #endif
#endif

etc...

[thinkyhead]

Only the E3D has the pins, so the way it is now is good.