iopoll: Introduce memory-mapped IO polling macros

It is sometimes necessary to poll a memory-mapped register until its value
satisfies some condition. Introduce a family of convenience macros that do
this. Tight-looping, sleeping, and timing out can all be accomplished using
these macros.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Robert Elliott <elliott@hp.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>