LoongArch: Fix CMDLINE_EXTEND and CMDLINE_BOOTLOADER handling

On FDT systems these command line processing are already taken care of
by early_init_dt_scan_chosen(). Add similar handling to the ACPI (non-
FDT) code path to allow these config options to work for ACPI (non-FDT)
systems too.

Signed-off-by: Zhihong Dong <donmor3000@hotmail.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>