Fix log spam about CPU frequency on one device

Previous kernels have allowed opening the CPU frequency file regardless
whether the CPU is up or not.  This fixes some log spam on one device
with dynamic hot plug CPU feature, which does not allow opening the CPU
frequency file if CPU is down.

Also, since the file descriptors are global and have long lives, add
the close-on-exec flag.

Change-Id: Ia14a2b9e20038dfb96a573920176a47a96bd3f5a