nrf52840: fix ram size

targets/circuitplay-bluefruit.ld

@@ -2,7 +2,7 @@
 MEMORY
 {
     FLASH_TEXT (rw) : ORIGIN = 0x00000000+0x26000, LENGTH = 0xED000-0x26000 /* SoftDevice S140. See https://learn.adafruit.com/introducing-the-adafruit-nrf52840-feather/hathach-memory-map. Application starts at 0x26000; user data starts at 0xED000 */
-    RAM (xrw)       : ORIGIN = 0x20004180, LENGTH = 37K
+    RAM (xrw)       : ORIGIN = 0x20004180, LENGTH = 0x20040000-0x20004180
 }
 
 _stack_size = 2K;