ARM: dts: beagle-x15: Add timers to IPU and DSP rprocs

Add the required system timers to all the remote processor device
nodes (IPU1, IPU2, DSP1 and DSP2) for the AM57xx Beagle X15 boards.
This data is identical to that of the equivalent processors on the
DRA7 EVM board to maintain firmware compatibility between both the
SoC and board variants.

The timers are optional, but are mandatory to support device
management features such as power management and watchdog support.
The above are added to successfully boot and execute firmware images
configured with the respective timers, images that use internal
processor subsystem timers are not affected. The timers can be
changed or removed as per the system integration needs, if needed,
after making the appropriate changes on the firmware side.

Each of the IPUs has two Cortex-M4 processors, and is currently
expected to be running in SMP-mode, so only a single timer suffices
to provide the BIOS tick timer. An additional timer should be added
for the second processor in IPU if it were to be run in non-SMP mode.
The timer value also needs to be unique from the ones used by other
processors so that they can be run simultaneously.

Signed-off-by: Suman Anna <s-anna@ti.com>