target/arm: Refactor handling of timer offset for direct register accesses

target/arm: Refactor handling of timer offset for direct register accesses

When reading or writing the timer registers, sometimes we need to
apply one of the timer offsets.  Specifically, this happens for
direct reads of the counter registers CNTPCT_EL0 and CNTVCT_EL0 (and
their self-synchronized variants CNTVCTSS_EL0 and CNTPCTSS_EL0).  It
also applies for direct reads and writes of the CNT*_TVAL_EL*
registers that provide the 32-bit downcounting view of each timer.

We currently do this with duplicated code in gt_tval_read() and
gt_tval_write() and a special-case in gt_virt_cnt_read() and
gt_cnt_read().  Refactor this so that we handle it all in a single
function gt_direct_access_timer_offset(), to parallel how we handle
the offset for indirect accesses.

The call in the WFIT helper previously to gt_virt_cnt_offset() is
now to gt_direct_access_timer_offset(); this is the correct
behaviour, but it's not immediately obvious that it shouldn't be
considered an indirect access, so we add an explanatory comment.

This commit should make no behavioural changes.

(Cc to stable because the following bugfix commit will
depend on this one.)

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20250204125009.2281315-6-peter.maydell@linaro.org
(cherry picked from commit 02c648a0a103a1a7b2c077ec5a81da9907f45544)
(Mjt: context fix in target/arm/internals.h)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>