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 <[e-mail]>
Closes: https://lore.kernel.org/lkml/[e-mail]/
Reviewed-by: Andy Shevchenko <[e-mail]>
Reviewed-by: Thomas Zimmermann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Javier Martinez Canillas <[e-mail]>

MAINTAINERS

···
 L:	[e-mail]
 S:	Maintained
 T:	git https://gitlab.freedesktop.org/drm/misc/kernel.git
+F:	drivers/firmware/sysfb*.c
 F:	drivers/gpu/drm/sysfb/
 F:	drivers/video/aperture.c
 F:	drivers/video/nomodeset.c
 F:	include/linux/aperture.h
+F:	include/linux/sysfb.h
 F:	include/video/nomodeset.h
 
 DRM DRIVER FOR GENERIC EDP PANELS