Convert CONFIG_SPL_SYS_MALLOC_SIMPLE to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_SYS_MALLOC_SIMPLE

The problem here is that a few platforms have been doing:
#ifdef CONFIG_SPL_BUILD
#define CONFIG_SYS_MALLOC_SIMPLE
#endif

instead of defining CONFIG_SPL_SYS_MALLOC_SIMPLE directly.  Correct this
and update the documentation in a few places to match usage.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/core/Kconfig b/drivers/core/Kconfig
index 408a8d8..27d6578 100644
--- a/drivers/core/Kconfig
+++ b/drivers/core/Kconfig
@@ -15,7 +15,7 @@
 	  Enable driver model in SPL. You will need to provide a
 	  suitable malloc() implementation. If you are not using the
 	  full malloc() enabled by CONFIG_SYS_SPL_MALLOC_START,
-	  consider using CONFIG_SYS_MALLOC_SIMPLE. In that case you
+	  consider using CONFIG_SPL_SYS_MALLOC_SIMPLE. In that case you
 	  must provide CONFIG_SPL_SYS_MALLOC_F_LEN to set the size.
 	  In most cases driver model will only allocate a few uclasses
 	  and devices in SPL, so 1KB should be enable. See
@@ -28,7 +28,7 @@
 	  Enable driver model in TPL. You will need to provide a
 	  suitable malloc() implementation. If you are not using the
 	  full malloc() enabled by CONFIG_SYS_SPL_MALLOC_START,
-	  consider using CONFIG_SYS_MALLOC_SIMPLE. In that case you
+	  consider using CONFIG_TPL_SYS_MALLOC_SIMPLE. In that case you
 	  must provide CONFIG_SPL_SYS_MALLOC_F_LEN to set the size.
 	  In most cases driver model will only allocate a few uclasses
 	  and devices in SPL, so 1KB should be enough. See
@@ -43,7 +43,7 @@
 	  Enable driver model in VPL. You will need to provide a
 	  suitable malloc() implementation. If you are not using the
 	  full malloc() enabled by CONFIG_SYS_SPL_MALLOC_START,
-	  consider using CONFIG_SYS_MALLOC_SIMPLE.
+	  consider using CONFIG_SPL_SYS_MALLOC_SIMPLE.
 
 config DM_WARN
 	bool "Enable warnings in driver model"