nrf/mpconfigport.h: Tune FAT FS configuration.

Disable MICROPY_FATFS_MULTI_PARTITION configuration because there is no
partition table in the flash for FATFS to read.

Also, set MICROPY_FATFS_MAX_SS to the size of a flash page.  For nrf51 the
value 1024 is set.  For nrf52/nrf91 the value 4096 is set.

ports/nrf/mpconfigport.h

@@ -75,7 +75,13 @@
 #define MICROPY_FATFS_LFN_CODE_PAGE    437 /* 1=SFN/ANSI 437=LFN/U.S.(OEM) */
 #define MICROPY_FATFS_USE_LABEL        (1)
 #define MICROPY_FATFS_RPATH            (2)
-#define MICROPY_FATFS_MULTI_PARTITION  (1)
+#define MICROPY_FATFS_MULTI_PARTITION  (0)
+
+#if NRF51
+    #define MICROPY_FATFS_MAX_SS       (1024)
+#else
+    #define MICROPY_FATFS_MAX_SS       (4096)
+#endif
 
 // TODO these should be generic, not bound to fatfs
 #define mp_type_fileio fatfs_type_fileio