ARM: 7930/1: Introduce atomic MMIO modify

Some SoC have MMIO regions that are shared across orthogonal
subsystems. This commit implements a possible solution for the
thread-safe access of such regions through a spinlock-protected API.

Concurrent access is protected with a single spinlock for the
entire MMIO address space. While this protects shared-registers,
it also serializes access to unrelated/unshared registers.

We add relaxed and non-relaxed variants, by using writel_relaxed and writel,
respectively. The rationale for this is that some users may not require
register write completion but only thread-safe access to a register.

Tested-by: Sebastian Hesselbarth <[email protected]>
Acked-by: Jason Cooper <[email protected]>
Acked-by: Catalin Marinas <[email protected]>
Signed-off-by: Ezequiel Garcia <[email protected]>
Signed-off-by: Russell King <[email protected]>

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