spl: serial_bcm283x: set GPIO before enabling uart to remove rx

spl: serial_bcm283x: set GPIO before enabling uart to remove rx

There was some garbage on uart input during boot, so if uart wasn't
connected, we end up in u-boot command line.
Set GPIO before enabling uart to remove uart Rx setup and this makes
SPL use only Tx.

Note: Chip documentation warns that order of commands is important.

Change-Id: I896d8b719c7b9170ad481767f8b27835d81f4af1
Signed-off-by: Rafal Tyminski <r.tyminski@partner.samsung.com>
[sw0312.kim: adjust commit-msg]
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>