riscv: remove @return doc for void functions

For functions returning nothing, there is no need to document
with @return, as Doxgen complains about "documented empty
return type of ...".

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

arch/riscv/core/prep_c.c

@@ -25,8 +25,6 @@
  * @brief Prepare to and run C code
  *
  * This routine prepares for the execution of and runs C code.
- *
- * @return N/A
  */
 
 void _PrepC(void)

arch/riscv/core/reboot.c

@@ -19,8 +19,6 @@
  * This is stub function to avoid build error with CONFIG_REBOOT=y
  * RISC-V specification does not have a common interface for system reset.
  * Each RISC-V SoC that has reset feature should implement own reset function.
- *
- * @return N/A
  */
 
 void __weak sys_arch_reboot(int type)

soc/riscv/esp32c3/idle.c

@@ -9,14 +9,11 @@
 #include <arch/cpu.h>
 
 /**
- *
  * @brief Power save idle routine
  *
  * This function will be called by the kernel idle loop or possibly within
  * an implementation of _pm_save_idle in the kernel when the
  * '_pm_save_flag' variable is non-zero.
- *
- * @return N/A
  */
 void arch_cpu_idle(void)
 {

soc/riscv/riscv-privilege/common/idle.c

@@ -22,14 +22,11 @@ static ALWAYS_INLINE void riscv_idle(unsigned int key)
 }
 
 /**
- *
  * @brief Power save idle routine
  *
  * This function will be called by the kernel idle loop or possibly within
  * an implementation of _pm_save_idle in the kernel when the
  * '_pm_save_flag' variable is non-zero.
- *
- * @return N/A
  */
 void arch_cpu_idle(void)
 {
@@ -37,7 +34,6 @@ void arch_cpu_idle(void)
 }
 
 /**
- *
  * @brief Atomically re-enable interrupts and enter low power mode
  *
  * INTERNAL
@@ -50,8 +46,6 @@ void arch_cpu_idle(void)
  *
  * 2) After waking up from the low-power mode, the interrupt lockout state
  *    must be restored as indicated in the 'imask' input parameter.
- *
- * @return N/A
  */
 void arch_cpu_atomic_idle(unsigned int key)
 {