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 <[e-mail]>
Signed-off-by: Geert Uytterhoeven <[e-mail]>