arm: mach-k3: am64x: Implement get_reset_reason()

arm: mach-k3: am64x: Implement get_reset_reason()

Implement get_reset_reason() for AM64x to enable reporting of the reset
cause in the cpuinfo output.

Notice that the AM64x does not support dedicated reset cause bits for
WDT and PORZ as the AM62x does.

An explanation of this difference is not part of the technical reference
manual and remains unclear.

Signed-off-by: Steffen Kothe <steffen.kothe@skothe.net>
Reviewed-by: Bryan Brattlof <bb@ti.com>