pinctrl: sh-pfc: r8a7794: Remove AVB_AVTP_* groups

The ATA_AVTP_* signals are documented as reserved in the recent R-Car E2
user's manual (the only remaining mention is in the table 5.2 and I believe
it's a simple overlook).  Remove the AVB_AVTP_* pinmux groups -- we will
remove the signals themselves in the next patch, along with the other now
reserved bits...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>