LoongArch: Support mem=<size> kernel parameter

The LoongArch mem= parameter parser was previously limited to the
mem=<size>@<start> format. This was inconvenient for the common use
case of simply capping the total system memory, as it forced users to
manually specify a start address. It was also inconsistent with the
behavior on other architectures.

This patch enhances the parser in early_parse_mem() to also support the
more user-friendly mem=<size> format. The implementation now checks for
the presence of the '@' symbol to determine the user's intent:

- If mem=<size> is provided (no '@'), the kernel now calls
  memblock_enforce_memory_limit(). This trims memory from the top down
  to the specified size.
- If mem=<size>@<start> is provided, the original behavior is retained
  for backward compatibility. This allows for defining specific memory
  banks.

This change introduces an important usage rule reflected in the code's
comments: the mem=<size> format should only be specified once on the
kernel command line. It acts as a single, global cap on total memory. In
contrast, the mem=<size>@<start> format can be specified multiple times
to define several distinct memory regions.

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