cpufreq: omap: Drop asm includes

The omap driver doesn't actually need asm/smp_plat.h, so drop it.
asm/cpu.h is not needed either as linux/cpu.h is already included.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>