clk: shmobile: div6: Make clock-output-names optional

Renesas DIV6 clocks provide a single clock output.  Hence make the
"clock-output-names" DT property optional instead of mandatory. In case
the DT property is omitted the DT node name will be used.

Rename the variable "name" to "clk_name" to make the code more similar
with fixed-factor-clock.c, and to avoid a conflict with a nested local
variable while we're at it.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>