arm: mach-k3: am62a: Add main_timer0 id to the dev list

main_timer0 is used by u-boot as the tick-timer. Add it to the soc
devices list so it an be enabled via the k3 power controller.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>

arm: mach-k3: Move R5 specific code into new r5/ directory

This makes it clear these are only to be used by the R5 builds of SPL.
And this will be used to later more cleanly split the two builds.

Signed-off-by: Andrew Davis <afd@ti.com>