arm: timer: Fix TVAL comparison for timer condition met

ARM DDI 0487G.a states on page D13-4180 that, when the virtual timer is
enabled, the timer condition is met when CNTVCT_EL0 - CNTV_CVAL_EL0 >= 0.
Multiplying both sides of the inequality by -1, we get the equivalent
condition CNTV_CVAL_EL0 - CNTVCT_EL0 <= 0 for when the timer should fire.

On the same page, it states that a read of the CNTV_TVAL_EL0 register
returns CNTV_CVAL_EL0 - CNTVCT_EL0 if the virtual timer is enabled.
Putting the two together, the timer condition is met when the value of the
TVAL register is less than or *equal* to 0.

Same rules apply for the physical timer.

Fix the check for the timer expiring by treating a TVAL value equal to zero
as a valid condition for the timer to fire.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Message-Id: <20211207154641.87740-2-alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>