minimize SD card reads when using SDCARD_SORT_ALPHA (Re-ARM board)

Because of the Re-ARM card's pinout there is only one SPI connected to
the RepRap Discount Full Graphic LCD display.  The LCD responds to ANY
SCK transitions no matter if it's enable is inactive.  The result is
garbage (usually bars) on the LCD display whenever there is SD card
activity.

This code minimizes this by only accessing the SD card when changing
directory levels if :
SDCARD_SORT_ALPHA is enabled
SDSORT_USES_RAM is true
SDSORT_CACHE_NAMES is true

The code changes result in file names being pulled from the ALPHA SORT
memory array rather than the SD card.

The code also gives the file count and file index functions their own
variables.  When they shared a common variable the index function
sometimes resulted in the file count being short by 1.

=======================================================================

cardreader.cpp & pins_RAMPS_RE_ARM.h changes

Added another condition to cardreader.cpp to enable getting file names
only from RAM.

pins_RAMPS_RE_ARM.h :
Added comments about the SD card accesses and the LCD display
Combined all versions into this one.

Marlin/cardreader.h

@@ -69,6 +69,8 @@ public:
   void updir();
   void setroot();
 
+  uint16_t get_num_Files();
+  
   #if ENABLED(SDCARD_SORT_ALPHA)
     void presort();
     void getfilename_sorted(const uint16_t nr);