ar71xx: rework chipidea controller support, add QCA9531

ar71xx: rework chipidea controller support, add QCA9531

Rework (again) platform support for dual-role chipidea USB controller:

- include support for QCA9531
- use correct EHCI block size
- drop ar933x_usb_setup_ctrl_config() function
- simplify code after previous "register chipidea only in device mode"
  change (fa22714181)

Reworked patch was tested on devices with below QCA WiSOCs (signal/GPIO
name with required bootstrap state for USB bus 0 in device mode):

- AR9331  (GPIO13 pull-down)
- AR9342  (RGMII_TXD1/ETXD1 pull-up)
- AR9344  (GPIO20 pull-up)
- QCA9531 (GPIO13 pull-up)
- QCA9558 (GPIO13 pull-up)

The only way to select device mode for bus 0 is to change SOC bootstrap
configuration which is sampled only once, at hard reset. Likely, other
models, like QCA9556 or AR9341, should also support dual-role USB mode
but they were not tested.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>