target/mips: Revert TARGET_PAGE_BITS_VARY

target/mips: Revert TARGET_PAGE_BITS_VARY

Revert ee3863b9d41 and a08d60bc6c2b.  The logic behind changing
the system page size because of what the Loongson kernel "prefers"
is flawed.

In the Loongson-2E manual, section 5.5, it is clear that the cpu
supports a 4k page size (along with many others).  Similarly for
the Loongson-3 series CPUs, the 4k page size is mentioned in the
section 7.7 (PageMask Register).  Therefore we must continue to
support a 4k page size.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250328175526.368121-2-richard.henderson@linaro.org>
[PMD: Mention Loongson-3 series CPUs]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
(cherry picked from commit fca2817fdcb00e65020c2dcfcb0b23b2a20ea3c4)
(Mjt: context fixes for 7.2.x)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>