Add the timer section of LoongArch in OSTD

ostd/src/arch/loongarch/timer/mod.rs

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: MPL-2.0
+
+//! The timer support.
+
+/// The timer frequency (Hz). Here we choose 1000Hz since 1000Hz is easier for
+/// unit conversion and convenient for timer. What's more, the frequency cannot
+/// be set too high or too low, 1000Hz is a modest choice.
+///
+/// For system performance reasons, this rate cannot be set too high, otherwise
+/// most of the time is spent executing timer code.
+pub const TIMER_FREQ: u64 = 1000;