am33xx: Remove serial_init in s_init for QSPI/NOR XIP boot

serial_init() reads global_data, since global_data is not yet
initialized, this can cause unwanted behaviour leading to QSPI XIP boot
hang. Also, since serial_init() is anyways called later from
boar_init_f(), it does not make sense to do the same in s_init().

Tested on AM437x IDK EVM with QSPI XIP boot.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>