rtc: Introduce ti-k3-rtc

Introduce support for Texas Instruments Real Time Clock controller on
newer K3 family of SoCs such as AM62x.

The hardware module that is being supported is the "digital only"
version which doesn't have capability of external wakeup sources and
external power backup. However, for many practical applications, this
should suffice as RTC is operational across low power sequences.

The hardware block by itself is split into two distinct domains
internally to further reduce the power consumption with the actual
counter block and comparators clocked off a 32k clock source (which
based on SoC integration can be sourced by an external crystal) and an
register interface block which is driven by the bus clock. While optimal
from power perspective, it does create some complicated synchronizations
and sequences that one must be wary of in the driver handling.

Acked-by: Andrew Davis <afd@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Tested-by: Georgi Vlaev <g-vlaev@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220623170808.20998-3-nm@ti.com