ARM: document and update UNCACHEABLE_ADDR definitions

Document the UNCACHEABLE_ADDR definitions for footbridge and SA1100
so that we know where they're located and/or what they're accessing.
Change RiscPC to calculate the UNCACHEABLE_ADDR value from FLUSH_BASE
as that's where we locate that.

UNCACHEABLE_ADDR is used to perform an uncached access (ARMv4
terminology) necessary to force a CPU clock-switch to the memory-
speed clock, as required for entering WFI.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>