This patch enables NO_HZ_IDLE (idle dynamic ticks) and HIGH_RES_TIMERS
(hrtimers) in RV32 and RV64 defconfigs.

Both of the above options are enabled by default for architectures
such as x86, ARM, and ARM64.

The idle dynamic ticks helps use save power by stopping timer ticks
when the system is idle whereas hrtimers is a much improved timer
subsystem compared to the old "timer wheel" based system.

This patch is tested on SiFive Unleashed board and QEMU Virt machine.
Signed-off-by: NAnup Patel <anup.patel@wdc.com>
Reviewed-by: NPalmer Dabbelt <palmer@sifive.com>
Signed-off-by: NPaul Walmsley <paul.walmsley@sifive.com>