vec: Support PAL-N.

It's supported as a standard preset within the VEC, but there was
no way of selecting it.
The 2 bits for SDTV mode in enum SDTV_MODE_T were fully utilised,
so add it as a modifier in one of the higher bits if PAL is
selected.

https://github.com/raspberrypi/firmware/issues/811

interface/vmcs_host/vc_sdtv.h

@@ -59,6 +59,8 @@ typedef enum SDTV_MODE_T_
 
    SDTV_MODE_PROGRESSIVE = 0x10, /**<240p progressive output*/
 
+   SDTV_MODE_PAL_N      = 0x40, /**<Use PAL-N instead of normal PAL*/
+
    SDTV_MODE_OFF        = 0x80,
    SDTV_MODE_FORCE_32BIT = 0x80000000
 } SDTV_MODE_T;