tty: amba-pl011: fix earlycon register offsets

The REG_x macros are indices into a table, not register offsets.  Since earlycon
does not have access to the vendor data, we can currently only support standard
ARM PL011 devices.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Huang Shijie <shijie.huang@arm.com>
Signed-off-by: Timur Tabi <timur@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>