rust: pl011: Cut down amount of text quoted from PL011 TRM

rust: pl011: Cut down amount of text quoted from PL011 TRM

Currently the comments in the Rust pl011 register.rs file include
large amounts of text from the PL011 TRM.  This is much more
commentary than we typically quote from a device reference manual,
and much of it is not relevant to QEMU.  Compress and rephrase the
comments so that we are not quoting such a large volume of TRM text.

We add a URL for the TRM; readers who need more detail on the
function of the register bits can find it there, presented in
context with the overall description of the hardware.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>