MIPS: make kgdb depend on FPU support

kgdb fails to build when the FPU support is disabled:

arch/mips/kernel/kgdb.c: In function 'dbg_set_reg':
arch/mips/kernel/kgdb.c:147:35: error: 'struct thread_struct' has no member named 'fpu'
  147 |    memcpy((void *)&current->thread.fpu.fcr31, mem,
      |                                   ^
arch/mips/kernel/kgdb.c:155:34: error: 'struct thread_struct' has no member named 'fpu'
  155 |   memcpy((void *)&current->thread.fpu.fpr[fp_reg], mem,

This is only relevant for CONFIG_EXPERT=y, so disallowing it
in Kconfig is an easier workaround than fixing it properly.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>