arm: add a very simple hotplugging driver for u8500 cpus

it checks for load between intervals and hot[un]plugs. it also hot-unplugs cpu1 on suspend and plugs it on resume. could be further improved.
it is made with u8500 in mind but i don't see how it can't be used for any dual-core phone

Change-Id: I2cdf0a49934ccbf90eeea9e3c4db012e5b38a5c1