clk: samsung: Fully constify mux parent names

The PNAME macro defines array of strings for names of mux parents.
Although the strings itself were const but pointers to them were not thus
this data resided in initdata. Make this an array of const pointers to
const strings and move to initconst section.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>