core/include: add macros for common units

I got tired of counting zeros in frequency defines, so add a few
helper macros to make defining frequency and sizes easier.

core/include/macros/units.h

@@ -0,0 +1,56 @@
+/*
+ * Copyright (C) 2020 ML!PA Consulting GmbH
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     core_macros
+ * @{
+ *
+ * @file
+ * @brief       Unit helper macros
+ *
+ * @author      Benjamin Valentin <benjamin.valentin@ml-pa.com>
+ */
+
+#ifndef MACROS_UNITS_H
+#define MACROS_UNITS_H
+
+#ifdef __cplusplus
+ extern "C" {
+#endif
+
+/**
+ * @brief   A macro to return the bytes in x KiB
+ */
+#define KiB(x) ((unsigned long long)(x) << 10)
+
+/**
+ * @brief   A macro to return the bytes in x MiB
+ */
+#define MiB(x) (KiB(x) << 10)
+
+/**
+ * @brief   A macro to return the bytes in x GiB
+ */
+#define GiB(x) (MiB(x) << 10)
+
+/**
+ * @brief   A macro to return the Hz in x kHz
+ */
+#define KHZ(x)    ((x) * 1000ULL)
+
+/**
+ * @brief   A macro to return the Hz in x MHz
+ */
+#define MHZ(x) (KHZ(x) * 1000ULL)
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* MACROS_UNITS_H */
+/** @} */