usb: r8a66597: Make CONFIG_RZA_USB default

No other platforms use this r8a66597 controller but RZ/A1,
make RZ/A1 support the default and drop all the other SoC
support to remove ifdeffery.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Chris Brandt <chris.brandt@renesas.com>