Have MBEDTLS_TIMING_C require MBEDTLS_HAVE_TIME

Nowadays, the timing module just builds on a function that provides a timer
with millisecond resolution. In terms of platform requirements, this is
almost exactly equivalent to `mbedtls_ms_time()`
provides (`mbedtls_ms_time()` is arguably a little stronger because it is
supposed to last longer than a single timer object, but an application could
start a timer when it starts, so there's no real difference.) So it's a bit
silly that `timing.c` essentially reimplements this. Rely on
`mbedtls_ms_time()` instead.

This is an API break because in Mbed TLS 4.0, it was possible to enable
`MBEDTLS_TIMING_C` without `MBEDTLS_HAVE_TIME`. However, `timing.c` only
provided an implementation for Windows and Unix-like platforms, and on those
platforms, it is very likely that the default implementation of
`MBEDTLS_HAVE_TIME` would also work. (The main exception would be a platform
that has the traditional Unix function `gettimeofday()`, but not the 1990s
novelty `clock_gettime()`.) So make this an official requirement, as a
belated change that really should have gone into 4.0 if we'd taken the time
to dig into it.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

ChangeLog.d/timing.txt

@@ -0,0 +1,5 @@
+API changes
+   * MBEDTLS_TIMING_C now requires MBEDTLS_HAVE_TIME to be enabled in the
+     TF-PSA-Crypto configuration, unless MBEDTLS_TIMING_ALT is enabled.
+     As a benefit, platforms where the default implementation is not
+     supported now only need to implement MBEDTLS_PLATFORM_MS_TIME_ALT.