extmod/mbedtls: Undefine ARRAY_SIZE if defined by platform.

This is an annoying regression caused by including mpconfig.h in 36922df -
the mimxrt platform headers define ARRAY_SIZE and mbedtls also defines in
some source files, using a different parameter name which is a warning in
gcc.

Technically mimxrt SDK is to blame here, but as this isn't a named warning
in gcc the only way to work around it in the mimxrt port would be to
disable all warnings when building this particular mbedTLS source file.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

extmod/mbedtls/mbedtls_config_common.h

@@ -123,4 +123,8 @@ void m_tracked_free(void *ptr);
 
 #endif
 
+// Workaround for a mimxrt platform driver header that defines ARRAY_SIZE,
+// which is also defined in some mbedtls source files.
+#undef ARRAY_SIZE
+
 #endif // MICROPY_INCLUDED_MBEDTLS_CONFIG_COMMON_H