arm: smh: Export semihosting functions

This exports semihosting functions for use in other files. The header is
in include/ and not arm/include/asm because I anticipate that RISC-V may
want to add their own implementation at some point.

smh_len_fd has been renamed to smh_flen to more closely match the
semihosting spec.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>

include/semihosting.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (C) 2022 Sean Anderson <sean.anderson@seco.com>
+ */
+
+#ifndef _SEMIHOSTING_H
+#define _SEMIHOSTING_H
+
+long smh_open(const char *fname, char *modestr);
+long smh_read(long fd, void *memp, size_t len);
+long smh_close(long fd);
+long smh_flen(long fd);
+
+#endif /* _SEMIHOSTING_H */