riscv32: timer: disable riscv_machine_timer driver by default for riscv32

Enabling the riscv_machine_timer driver by default for riscv32
causes compilation issues on riscv32 boards (like zedboard_pulpino)
not supporting it.

Boards supporting the driver enable it via their respective
config file.

Change-Id: Ieb0d25fa339834fd386ae2725f40b6b7b72dc52b
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>