remoteproc: keystone: Use devm_kasprintf() to build name string

This is simpler and removes the need to assume the id length to be 1
digit, which then removes a W=1 compile warning about the same.

Signed-off-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20240802182300.244055-1-afd@ti.com


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>