iopoll: import include/linux/iopoll.h from Linux 4.9

This was imported from Linux 4.9 and adjusted for U-Boot.

- Replace the license block with SPDX
- Drop all *_atomic variants, which make no se

iopoll: import include/linux/iopoll.h from Linux 4.9

This was imported from Linux 4.9 and adjusted for U-Boot.

- Replace the license block with SPDX
- Drop all *_atomic variants, which make no sense for U-Boot
- Remove the sleep_us argument, which makes no sense for U-Boot

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

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