drivers: timers: riscv: don't leak Kconfig symbols
drivers: timers: riscv: don't leak Kconfig symbols

Add if check around RISC-V machine specific kconfig symbols so they
aren't exposed to everyone.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@intel.com>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr