MIPS: Loongson64: Give chance to build under !CONFIG_NUMA and !CONFIG_SMP

In the current code, we can not build under !CONFIG_NUMA and !CONFIG_SMP
on the Loongson64 platform, it seems bad for the users who just want to
use pure single core (not nosmp) to debug, so do the following things to
give them a chance:

(1) Do not select NUMA and SMP for MACH_LOONGSON64 in Kconfig, make NUMA
depends on SMP, and then just set them in the loongson3_defconfig.
(2) Move szmem() from numa.c to init.c and add prom_init_memory() under
!CONFIG_NUMA.
(3) Clean up szmem() due to the statements of case SYSTEM_RAM_LOW and
SYSTEM_RAM_HIGH are the same.
(4) Remove the useless declaration of prom_init_memory() and add the
declaration of szmem() in loongson.h to avoid build error.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>