mips: BCM63XX: ensure that CPU_SUPPORTS_32BIT_KERNEL is set

Several header files need info on CONFIG_32BIT or CONFIG_64BIT,
but kconfig symbol BCM63XX does not provide that info. This leads
to many build errors, e.g.:

   arch/mips/include/asm/page.h:196:13: error: use of undeclared identifier 'CAC_BASE'
           return x - PAGE_OFFSET + PHYS_OFFSET;
   arch/mips/include/asm/mach-generic/spaces.h:91:23: note: expanded from macro 'PAGE_OFFSET'
   #define PAGE_OFFSET             (CAC_BASE + PHYS_OFFSET)
   arch/mips/include/asm/io.h:134:28: error: use of undeclared identifier 'CAC_BASE'
           return (void *)(address + PAGE_OFFSET - PHYS_OFFSET);
   arch/mips/include/asm/mach-generic/spaces.h:91:23: note: expanded from macro 'PAGE_OFFSET'
   #define PAGE_OFFSET             (CAC_BASE + PHYS_OFFSET)

arch/mips/include/asm/uaccess.h:82:10: error: use of undeclared identifier '__UA_LIMIT'
           return (__UA_LIMIT & (addr | (addr + size) | __ua_size(size))) == 0;

Selecting the SYS_HAS_CPU_BMIPS* symbols causes SYS_HAS_CPU_BMIPS to be
set, which then selects CPU_SUPPORT_32BIT_KERNEL, which causes
CONFIG_32BIT to be set. (a bit more indirect than v1 [RFC].)

Fixes: e7300d04 ("MIPS: BCM63xx: Add support for the Broadcom BCM63xx family of SOCs.")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: linux-mips@vger.kernel.org
Cc: Paul Burton <paulburton@kernel.org>
Cc: Maxime Bizon <mbizon@freebox.fr>
Cc: Ralf Baechle <ralf@linux-mips.org>
Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>