ARM: omap2: make arrays containing machine compatible strings const

The definition

	static const char *omap3_boards_compat[] __initconst = {

defines a changable array of constant strings. That is you must not do:

	*omap3_boards_compat[0] = 'f';

but

	omap3_boards_compat[0] = "another string";

is fine. So the annotation __initconst is wrong and yields a compiler
error when other really const variables are added with __initconst.

As the struct machine_desc member dt_compat is declared as

	const char *const *dt_compat;

making the arrays const is the better alternative over changing all
annotations to __initdata.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>