remoteproc: k3-r5: Use separate compatible string for TI AM62x SoC family

AM62 and AM62A SoCs use single core R5F which is a new scenario
different than the one being used with CLUSTER_MODE_SINGLECPU which is
for utilizing a single core from a set of cores available in R5F cluster
present in the SoC.

To support this single core scenario map it with newly defined
CLUSTER_MODE_SINGLECORE and use it when compatible is set to
ti,am62-r5fss.

Also set PROC_BOOT_CFG_FLAG_R5_SINGLE_CORE config for
CLUSTER_MODE_SINGLECORE too as it is required by R5 core when it is
being as general purpose core instead of device manager.

For IPC-only mode when config flag PROC_BOOT_CFG_FLAG_R5_SINGLE_CORE
is set, skip overriding to CLUSTER_MODE_SINGLECPU when
CLUSTER_MODE_SINGLECORE is set as same flag is applicable for single
core configuration too.

Signed-off-by: Devarsh Thakkar <devarsht@ti.com>
Link: https://lore.kernel.org/r/20230327152832.923480-4-devarsht@ti.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>