riscv32: timer: replace riscv_qemu_driver by the generic riscv_machine_driver

riscv defines the machine-mode timer registers that are implemented
by the all riscv SOCs that follow the riscv privileged architecture
specification.

The timer registers implemented in riscv-qemu follow this specification.
To account for future riscv SOCs, reimplement the riscv_qemu_driver by
the riscv_machine_driver.

Change-Id: I645b03c91b4e07d0f2609908decc27ba9b8240d4
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>