clk: samsung: Modify _get_rate() helper to use __clk_lookup()

There is no need to use clkdev inside the clock driver to retrieve the
clocks for internal use. Instead __clk_lookup() helper can be used to
look up clocks by their platform name.

This patch modifies the behavior of _get_rate() helper to look up clocks
by platform name and adjusts all users of it to pass platform names
instead of clkdev aliases.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>