ramips: mt7621: add support for memory detection

mt7621 has the following memory map:
0x0-0x1c000000: lower 448m memory
0x1c000000-0x2000000: peripheral registers
0x20000000-0x2400000: higher 64m memory

detect_memory_region in arch/mips/kernel/setup.c only add the first
memory region and isn't suitable for 512m memory detection because
it may accidentally read the memory area for peripheral registers.

This commit adds memory detection capability for mt7621:
1. add the highmem area when 512m is detected.
2. guard memcmp from accessing peripheral registers:
     This only happens when some weird user decided to change
     kernel load address to 256m or higher address. Since this
     is a quite unusual case, we just skip 512m testing and return
     256m as memory size.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>