ARM: 6421/1: amba-pl011: add missing ST specific registers

The ST Micro derivates have several extra interesting registers
that we may soon use for something interesting so may just as
well define them in the header.

Signed-off-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>