MAINTAINERS: Add missing sysfb files to firmware framebuffers entry

MAINTAINERS: Add missing sysfb files to firmware framebuffers entry

The commit d391c5827107 ("drivers/firmware: move x86 Generic System
Framebuffers support") moved the sysfb*.c source files from arch/x86
to drivers/firmware, because the logic wasn't x86 specific and could
be used by other architectures.

But the drivers/firmware path is not listed in MAINTAINERS, which led
to the files being orphaned and scripts/get_maintainer.pl not listing
a mailing list to Cc anymore.

Now that we have an entry for all the firmware-provided framebuffer code,
add the missing sysfb files to make sure correct folks and list is Cc'ed.

Fixes: d391c5827107 ("drivers/firmware: move x86 Generic System Framebuffers support")
Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Closes: https://lore.kernel.org/lkml/aF53djlieUNF_-aV@smile.fi.intel.com/
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250627113328.2703491-1-javierm@redhat.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>