ARM: s3c64xx: Annotate external clock frequencies __ro_after_init

The xtal_f and xusbxti_f static variables are modified only through
__init accessors (like s3c64xx_set_xtal_freq()).  Later these variables
are used only in read-only way so we can mark them __ro_after_init to
increase code safeness.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>