clk: renesas: mstp: Remove obsolete clkdev registration

After the DT conversion of SH-Mobile and Armadillo-800-EVA display
support, all devices are registered from DT, so we can remove the
registration of clkdevs.

Add the missing #include <linux/slab.h>, which was included implicitly
through <linux/clkdev.h> before.

Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/e98a6e47ebecc44fa41de6d88b4ed20c6efbd177.1705931322.git.geert+renesas@glider.be