riscv: provide MODIFIES for read_sip

read_sip does not modify any state

Signed-off-by: Rafal Kolanski <rafal.kolanski@data61.csiro.au>

include/arch/riscv/arch/machine.h

@@ -152,6 +152,7 @@ static inline word_t read_sstatus(void)
     return temp;
 }
 
+/** MODIFIES: */
 static inline word_t read_sip(void)
 {
     word_t temp;