usb: gadget: f_fs: add usb_functionfs_descs_head_v2 structure

The structure can be used with user space tools that use the new
functionfs description format, for example as follows:

static const struct {
	struct usb_functionfs_descs_head_v2 header;
	__le32 fs_count;
	__le32 hs_count;
	struct {
		…
	} fs_desc;
	struct {
		…
	} hs_desc;
} descriptors = {
	.header = {
		.magic = cpu_to_le32(FUNCTIONFS_DESCRIPTORS_MAGIC_V2),
		.length = cpu_to_le32(sizeof(descriptors)),
		.flags = cpu_to_le32(FUNCTIONFS_HAS_FS_DESC |
				     FUNCTIONFS_HAS_HS_DESC)
	},
	.fs_count = cpu_to_le32(X),
	.fs_desc = {
		…
	},
	.hs_count = cpu_to_le32(Y),
	.hs_desc = {
		…
	}
};

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>