riscv/riscv_fpu: Clear the full FPU status/control register on boot

Instead of clearing the fields individually, just wipe the whole register.
This can be done because flags and rm are just parts of the fcsr.

31             8        5           0
+--------------+--------+-----------+
|              |        |           |
|   RESERVED   |  FRM   |  FSTATUS  |
|              |        |           |
+--------------+--------+-----------+
                FCSR

arch/risc-v/src/common/riscv_fpu.S

@@ -69,8 +69,7 @@ riscv_fpuconfig:
   li           a0, MSTATUS_FS_INIT
   csrs         CSR_STATUS, a0
 
-  fsflags      zero
-  fsrm         zero
+  fscsr        zero
 
   fence.i
   ret