clk: si5351: Constify clock names and struct regmap_config

The regmap_config struct may be const because it is not modified by the
driver and regmap_init() accepts pointer to const.

Replace doubled const in the arrays of clock names with proper const
pointer to const data. This fixes the warnings:

drivers/clk/clk-si5351.c:71:25: warning: duplicate const
drivers/clk/clk-si5351.c:74:25: warning: duplicate const
drivers/clk/clk-si5351.c:77:25: warning: duplicate const
drivers/clk/clk-si5351.c:80:25: warning: duplicate const

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>