doc: arch: sandbox: Replace all the instances of README.sandbox

commit 49116e6d236d ("doc: arch: Convert README.sandbox to reST")
Moves README.sandbox to doc/arch.
Replace all the existing instances to point to the right documentation
file.

Signed-off-by: Keerthy <j-keerthy@ti.com>
diff --git a/arch/sandbox/include/asm/spi.h b/arch/sandbox/include/asm/spi.h
index 9985e3c..98e1826 100644
--- a/arch/sandbox/include/asm/spi.h
+++ b/arch/sandbox/include/asm/spi.h
@@ -1,5 +1,5 @@
 /*
- * Simulate a SPI port and clients (see README.sandbox for details)
+ * Simulate a SPI port and clients (see doc/arch/sandbox.rst for details)
  *
  * Copyright (c) 2011-2013 The Chromium OS Authors.
  * See file CREDITS for list of people who contributed to this