ramips: mt7621: do memory detection on KSEG1

ramips: mt7621: do memory detection on KSEG1

It's reported that current memory detection code occasionally detects
larger memory under some bootloaders.
Current memory detection code tests whether address space wraps around
on KSEG0, which is unreliable because it's cached.

Rewrite memory size detection to perform the same test on KSEG1 instead.
While at it, this patch also does the following two things:
1. use a fixed pattern instead of a random function pointer as the magic
   value.
2. add an additional memory write and a second comparison as part of the
   test to prevent possible smaller memory detection result due to
   leftover values in memory.

Fixes: 6d91ddf517 ("ramips: mt7621: add support for memory detection")
Reported-by: Rui Salvaterra <rsalvaterra@gmail.com>
Tested-by: Rui Salvaterra <rsalvaterra@gmail.com>
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
(cherry picked from commit 2f024b79331141e2a62c9bf3601c803b26bde77b)
[backport for OpenWrt 21.02 as it was reproducible with Kernel 5.4, see [1]]
[1]: https://forum.openwrt.org/t/113081
Tested-by: Dimitri Souza <dimitri.souza@gmail.com> [mt7621/archer-c6-v3]
Signed-off-by: Szabolcs Hubai <szab.hu@gmail.com>