Add getclksfreq(), wfe(), wfi(), rnd.setup() and rnd.read() functions.

wfe() and wfi() just call the underlying ARM functions.

getclksfreq() returns a table containing these clock frequencies: SYSCLK,
HCLK, PCLK1 and PCLK2.

rnd.setup() initialises the random number generator. rnd.read() will call
this anyway if the RNG hasn't been initialised before you read from it. If
rng.read() ever returns nil to signify a problem, you can call rnd.setup()
to re-initialise the RNG.

rnd.read() returns a 32 bit random number or nil if the generator signals
an error.