ARM: imx: clk-imx6sl: Suppress duplicate const sparse warning

There should be no duplicate const specifiers for those static
constant character string arrays defined for clock mux options.
Also, the arrays are only taken as the 5th argument for the
imx_clk_mux() function, which is in the type of 'const char
**parents'.  So, let's remove the 2nd const specifier right
after 'char'.

This patch fixes these sparse warnings:
arch/arm/mach-imx/clk-imx6sl.c:21:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:22:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:23:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:24:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:25:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:26:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:27:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:28:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:29:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:30:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:31:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:32:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:33:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:34:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:35:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:36:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:37:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:38:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:39:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:40:25: warning: duplicate const
arch/arm/mach-imx/clk-imx6sl.c:41:25: warning: duplicate const

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>