media: atmel: fix build when ISC=m and XISC=y

Building VIDEO_ATMEL_ISC as module and VIDEO_ATMEL_XISC as built-in
(or viceversa) causes build errors:

 or1k-linux-ld: drivers/media/platform/atmel/atmel-isc-base.o: in function `isc_async_complete':
 atmel-isc-base.c:(.text+0x40d0): undefined reference to `__this_module'
 or1k-linux-ld: atmel-isc-base.c:(.text+0x40f0): undefined reference to `__this_module'
 or1k-linux-ld: drivers/media/platform/atmel/atmel-isc-base.o:(.rodata+0x390): undefined reference to `__this_module'
 or1k-linux-ld: drivers/media/platform/atmel/atmel-isc-base.o:(__param+0x4): undefined reference to `__this_module'
 or1k-linux-ld: drivers/media/platform/atmel/atmel-isc-base.o:(__param+0x18): undefined reference to `__this_module'

This is caused by the file atmel-isc-base.c which is common code between
the two drivers.

The solution is to create another Kconfig symbol that is automatically
selected and generates the module atmel-isc-base.ko. This module can be
loaded when both drivers are modules, or built-in when at least one of them
is built-in.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: c9aa9738 ("media: atmel: atmel-isc: add microchip-xisc driver")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>