MIPS: Loongson64: Fix up reserving kernel memory range

Reserve memory from &_text to &_end. Otherwise if kernel address
was modified, the memory range of start_pfn to kernel_start_pfn
would be reserved. Then we could not use this range.

Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>