clk: samsung: Use cached clk_hws instead of __clk_lookup() calls

clk: samsung: Use cached clk_hws instead of __clk_lookup() calls

For the CPU clock registration two parent clocks are required, these
are now being passed as struct clk_hw pointers, rather than by the
global scope names. That allows us to avoid  __clk_lookup() calls
and simplifies a bit the CPU clock registration function.
While at it drop unneeded extern keyword in the function declaration.

Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Link: https://lore.kernel.org/r/20200826171529.23618-3-s.nawrocki@samsung.com
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>