dt-bindings: clk: emev2: Rename bindings documentation file

Rename the device tree clock bindings for Renesas EMMA Mobile EV2
from emev2-clock.txt to renesas,emev2-smu.txt.

This is part of an ongoing effort to name bindings documentation files for
Renesas IP blocks consistently, in line with the compat strings they
document.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>