ARM: Remove unnecessary subpage workarounds

In the ARM per-CPU peripherals (GIC, private timers, SCU, etc),
remove workarounds for subpage memory region read/write functions
being passed offsets from the start of the page rather than the
start of the region. Following commit 5312bd8b the masking off
of high bits of the address offset is now harmless but unnecessary.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>