video: fbdev: vt8623fb: Constify static vga_regsets

These are not modified so make them const to allow the compiler to put
them in read-only memory.

Before:
   text    data     bss     dec     hex filename
  25509    7928      64   33501    82dd drivers/video/fbdev/vt8623fb.o

After:
   text    data     bss     dec     hex filename
  26533    6904      64   33501    82dd drivers/video/fbdev/vt8623fb.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200701210248.64893-1-rikard.falkeborn@gmail.com