ARM: shmobile: rcar-gen2: Make rcar_gen2_{timer_init, reserve}() static

As of commit 1e90fea3 ("ARM: shmobile: r8a7791: Use common R-Car
Gen2 machine definition"), there are no more users of
rcar_gen2_timer_init() and rcar_gen2_reserve() outside
arch/arm/mach-shmobile/setup-rcar-gen2.c.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200908074403.4379-1-geert+renesas@glider.be