remoteproc/davinci: fix quoted split string checkpatch warning

Fix the following checkpatch warning,
WARNING: quoted string split across lines
+                "\n\t\tName of DSP firmware file in /lib/firmware"
+                " (if not specified defaults to 'rproc-dsp-fw')");

Signed-off-by: Suman Anna <s-anna@ti.com>
[remove leading whitespace as well]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>