doc: Fix misuse of @file doxygen tag

Summary lines follow the @brief tag instead of the @file tag.

This prevents doxygen build warnings such as ...

    The name xxx supplied as the second argument in the \file
    statement is not an input file

Change-Id: I1014586ad21be12e14aa1d2a942e6b8a11211795
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>

include/drivers/serial/uart_nsim.h

@@ -15,7 +15,8 @@
  */
 
 /**
- * @file Header file for NSIM UART driver
+ * @file
+ * @brief Header file for NSIM UART driver
  */
 
 #ifndef _DRIVERS_UART_NSIM_H_

include/irq_offload.h

@@ -15,7 +15,8 @@
  */
 
 /**
- * @file IRQ Offload interface
+ * @file
+ * @brief IRQ Offload interface
  */
 #ifndef _IRQ_OFFLOAD_H_
 #define _IRQ_OFFLOAD_H_

lib/libc/minimal/include/bits/null.h

@@ -15,7 +15,8 @@
  */
 
 /**
- * @file NULL definition
+ * @file
+ * @brief NULL definition
  */
 
 #if !defined(NULL)

lib/libc/minimal/include/bits/restrict.h

@@ -15,7 +15,8 @@
  */
 
 /**
- * @file _Restrict definition
+ * @file
+ * @brief _Restrict definition
  *
  * The macro "_Restrict" is intended to be private to the minimal libc library.
  * It evaluates to the "restrict" keyword when a C99 compiler is used, and

lib/libc/minimal/include/bits/size_t.h

@@ -15,7 +15,8 @@
  */
 
 /**
- * @file size_t definition
+ * @file
+ * @brief size_t definition
  */
 
 #if !defined(__size_t_defined)