TCL scripts: collect duplicated procedures

TCL procedures mrw and mmw, originally in DaVinci target code,
are duplicated in other TCL scripts.
Moved in a common helper file, and added help/usage description.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>

tcl/target/davinci.cfg

@@ -7,20 +7,7 @@ proc davinci_pinmux {soc reg value} {
 	mww [expr [dict get $soc sysbase] + 4 * $reg] $value
 }
 
-# mrw: "memory read word", returns value of $reg
-proc mrw {reg} {
-	set value ""
-	mem2array value 32 $reg 1
-	return $value(0)
-}
-
-# mmw: "memory modify word", updates value of $reg
-#	$reg <== ((value & ~$clearbits) | $setbits)
-proc mmw {reg setbits clearbits} {
-	set old [mrw $reg]
-	set new [expr ($old & ~$clearbits) | $setbits]
-	mww $reg $new
-}
+source [find mem_helper.tcl]
 
 #
 # pll_setup: initialize PLL