doc: drop references to non-existing CONFIG_MEM_SUPPORT_64BIT_DATA

Such a config option does not exist. Rephrase, and avoid mentioning
MEM_SUPPORT_64BIT_DATA, which is an implementation detail.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
diff --git a/doc/usage/cmd/cmp.rst b/doc/usage/cmd/cmp.rst
index 8d196ee..66865eb 100644
--- a/doc/usage/cmd/cmp.rst
+++ b/doc/usage/cmd/cmp.rst
@@ -96,7 +96,7 @@
 -------------
 
 The cmp command is only available if CONFIG_CMD_MEMORY=y. The cmp.q command is
-only available if additionally CONFIG_MEM_SUPPORT_64BIT_DATA=y.
+only available on 64-bit targets.
 
 Return value
 ------------
diff --git a/doc/usage/cmd/cp.rst b/doc/usage/cmd/cp.rst
index 67360e3..bea379f 100644
--- a/doc/usage/cmd/cp.rst
+++ b/doc/usage/cmd/cp.rst
@@ -74,7 +74,7 @@
 -------------
 
 The cp command is available if CONFIG_CMD_MEMORY=y. Support for 64 bit words
-(cp.q) depends on CONFIG_MEM_SUPPORT_64BIT_DATA=y. Copying to flash depends on
+(cp.q) is only available on 64-bit targets. Copying to flash depends on
 CONFIG_MTD_NOR_FLASH=y.
 
 Return value