drm/mgag200: Declare PLL clock constants static const

Move the PLL constants to the RO data section by declaring them as
static const. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714142240.21979-10-tzimmermann@suse.de