arm, am33xx: move uart soft reset code to common place

move uart soft reset code to common place and call
this function from board code, instead of copy and paste
this code for every board.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Matt Porter <mporter@ti.com>
Cc: Lars Poeschel <poeschel@lemonage.de>
Cc: Tom Rini <trini@ti.com>
Cc: Enric Balletbo i Serra <eballetbo@iseebcn.com>
Acked-by: Tom Rini <trini@ti.com>
[trini: Fix igep0033 build, remove 'regval' on pcm051]
Signed-off-by: Tom Rini <trini@ti.com>