MIPS: Sibyte: Include the swarm subdir to the sb1250 LittleSur builds

MIPS: Sibyte: Include the swarm subdir to the sb1250 LittleSur builds

Fixes the following randconfig build problem:

arch/mips/built-in.o: In function `show_cpuinfo':
proc.c:(.text+0xde84): undefined reference to `get_system_type'
arch/mips/built-in.o: In function `sb1250_setup':
(.init.text+0x428): undefined reference to `get_system_type'
arch/mips/built-in.o: In function `setup_arch':
(.init.text+0x178c): undefined reference to `plat_mem_setup'
Makefile:930: recipe for target 'vmlinux' failed

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8106/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>