ARM: clps711x: mark clps711x_compat as const

The array of string pointers is put in __initconst, and the strings themselves
are marke 'const' but the the pointers are not, which caused a warning when
built with LTO:

arch/arm/mach-clps711x/board-dt.c:72:20: error: 'clps711x_compat' causes a section type conflict with 'feroceon_ids'
 static const char *clps711x_compat[] __initconst = {

This marks the array itself const as well, which was certainly the
intention originally.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>