arm: dts: k3-j721e: Fix up MAIN R5FSS cluster mode back to Split-mode

The default U-Boot environment variables and design are all set up for
both the MAIN R5FSS clusters to be in Split-mode. This is the setting
in v2021.01 U-Boot and the dt nodes are synched with the kernel binding
property names in commit 468ec2f3 ("remoteproc: k3_r5: Sync to
upstreamed kernel DT property names") merged in v2021.04-rc2.

The modes for both the clusters got switched back to LockStep mode by
mistake in commit 70e16749 ("arm: dts: k3-j721e: Sync Linux v5.11-rc6
dts into U-Boot") also in v2021.04-rc2. This throws the following warning
messages when early-booting the cores using default env variables,

  k3_r5f_rproc r5f at 5d00000: Invalid op: Trying to start secondary core 7 in lockstep mode
  Load Remote Processor 3 with data at addr=0x82000000 98484 bytes: Failed!
  k3_r5f_rproc r5f at 5f00000: Invalid op: Trying to start secondary core 9 in lockstep mode
  Load Remote Processor 5 with data at addr=0x82000000 98484 bytes: Failed!

Fix this by switching back both the clusters to the expected Split-mode.
Make this mode change in the u-boot specific dtsi file to avoid such
sync overrides in the future until the kernel dts is also switched to
Split-mode by default.

Fixes: 70e16749 ("arm: dts: k3-j721e: Sync Linux v5.11-rc6 dts into U-Boot")
Reported-by: Minas Hambardzumyan <minas@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>