usb: gadget: aspeed: add ast2600 vhub support

Add AST2600 support in aspeed-vhub driver. There are 3 major differences
between AST2500 and AST2600 vhub:
  - AST2600 supports 7 downstream ports while AST2500 supports 5.
  - AST2600 supports 21 generic endpoints while AST2500 supports 15.
  - EP0 data buffer's 8-byte DMA alignment restriction is removed from
    AST2600.

Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Felipe Balbi <balbi@kernel.org>