ubb-vga: try to clock MSC faster than the MMC bus; new video mode "640x480/61"

- ubb-vga.c (mode_db): added new mode "640x480/61"
- ubb-vga.c (setup, line): MSC_CLKRT is reset when the MSC resets. Moved
  it from "setup" to "line".
- ubb-vga.c (clkrt, setup, line): pick the highest available MSC clock
  and adjust the bus clock divider accordingly
- ubb-vga.c (main): added commented-out clock debugging output
- README: added XEN-1510T result for "640x480/61"

ubb-vga/README

@@ -48,6 +48,8 @@ Mode		Monitor		Image   Hor. noise	Cleanliness
 		W2243C		y y	2-4   large	very distorted
 		W2243L		y y	2-3   large	very distorted
 
+640x480/61	XEN-1510T	y y	1     -/10, 20%	very good/distorted (5)
+
 640x480/70	XEN-1510T	y (2)	1     10, 25%	distorted, flickery
 		LN32R71B	n -	"not supported mode"
 		206NW		y (2)	3-8   -		very jittery
@@ -115,6 +117,8 @@ that the screen content cannot be recognized.
     manually.
 (4) Places the image too far to the left. Cannot be sufficiently
     corrected.
+(5) about 70% of all session have a clean stable image. The remaining
+    30% suffer significant FIFO noise.
 
 
 To do