Increase max V. Backporch value from 63 to 236

This change allows highly letterboxed content (e.g. the PSP's 480x272
picture in a 720x480 frame) to be "zoomed" to a full screen picture by
treating the letterbox as horizontal and vertical backporch.

Co-authored-by: Chris Lockfort <clockfort@gmail.com>

ip/sc_config/inc/sc_config_regs.h

@@ -81,10 +81,10 @@ typedef union {
 typedef union {
     struct {
         alt_u16 v_active:11;
-        alt_u8 v_backporch:6;
+        alt_u8 v_backporch:8;
         alt_u8 v_synclen:3;
         alt_u8 v_mask:6;
-        alt_u8 v_rsv:3;
+        alt_u8 v_rsv:1;
         alt_u8 v_multmode:3;
     } __attribute__((packed, __may_alias__));
     alt_u32 data;