LoongArch: Reserve the EFI memory map region

The EFI memory map at 'boot_memmap' is crucial for kdump to understand
the primary kernel's memory layout. This memory region, typically part
of EFI Boot Services (BS) data, can be overwritten after ExitBootServices
if not explicitly preserved by the kernel.

This commit addresses this by:
1. Calling memblock_reserve() to reserve the entire physical region
   occupied by the EFI memory map (header + descriptors). This prevents
   the primary kernel from reallocating and corrupting this area.
2. Setting the EFI_PRESERVE_BS_REGIONS flag in efi.flags. This indicates
   that efforts have been made to preserve critical BS code/data regions
   which can be useful for other kernel subsystems or debugging.

These changes ensure the original EFI memory map data remains intact,
improving kdump reliability and potentially aiding other EFI-related
functionalities that might rely on preserved BS code/data.

Signed-off-by: Ming Wang <wangming01@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>