USB: serial: xr: add support for XR22801, XR22802, XR22804

The XR22801, XR22802 and XR22804 are compound devices with an embedded
hub and up to seven downstream USB devices including one, two or four
UARTs respectively.

The UART function is similar to XR21B142X but most registers are offset
by 0x40, the register requests are different and are directed at the
device rather than interface, and 5 and 6-bit words are not supported.

Signed-off-by: Johan Hovold <johan@kernel.org>