barebox adoption of Linux commit:

| commit 5f5323a
| Author: Dejin Zheng <zhengdejin5@gmail.com>
| Date:   Mon Mar 23 23:05:51 2020 +0800
|
|     iopoll: introduce read_poll_timeout macro
|
|     this macro is an extension of readx_poll_timeout macro. the accessor
|     function op just supports only one parameter in the readx_poll_timeout
|     macro, but this macro can supports multiple variable parameters for
|     it. so functions like phy_read(struct phy_device *phydev, u32 regnum)
|     and phy_read_mmd(struct phy_device *phydev, int devad, u32 regnum) can
|     also use this poll timeout core. and also expand it can sleep some time
|     before read operation.
|
|     Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
|     Signed-off-by: David S. Miller <davem@davemloft.net>

Also, implement readx_poll_timeout using read_poll_timeout.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>