Juno: Introduce juno_getentropy(void *buf, size_t len)

Juno: Introduce juno_getentropy(void *buf, size_t len)

This function fills the buffer (first argument) with the specified
number of bytes (second argument) from the trusted entropy source.

This function will be used to initialize the stack protector canary.

Change-Id: Iff15aaf4778c13fa883ecb5528fcf9b8479d4489
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>