riscv: make mcycle & minstret high bits writable

Author: hegza

riscv/src/register/mcycleh.rs

@@ -1,3 +1,4 @@
 //! mcycleh register
 
 read_csr_as_usize_rv32!(0xB80);
+write_csr_as_usize_rv32!(0xB80);

riscv/src/register/minstreth.rs

@@ -1,3 +1,4 @@
 //! minstreth register
 
 read_csr_as_usize_rv32!(0xB82);
+write_csr_as_usize_rv32!(0xB82);