serial: pl011: fix building conflict caused by upgrading kernel

After upgrading kernel, the commit 211565b1("serial: pl011:
UPSTAT_AUTORTS requires .throttle/unthrottle") is involved from
mainline upstream. But it confilts with previous commit 0f0f7d7e
("tty: amba-pl011: Add un/throttle support") that is from SDK repo.
So, remove the SDK code and keep the upstream code to avoid the
building issue.

Signed-off-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>