ITE: soc/riscv/ite: policy: Add minimum residency time to enter sleep

Adding this condition will limit the minimum residency time to enter
sleep mode. This will fix tests in test\kernel\sleep\usleep.c causing
longer than expected test times due to going into sleep mode with no
time limit.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>