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TX FIFO has space for 32 characters. With default UART baudrate 115200 it
takes more than 2ms to transmit all 32 characters, so wait at least 3ms
before flushing TX FIFO.

If WTMI firmware transmitted something via UART before TF-A was booted,
some characters may still wait in TX FIFO when TF-A is initializing UART
driver. So wait at least 3ms to ensure that HW has enough time to transmit
all characters waiting in TX FIFO.

This fixes an issue where sometimes characters transmitted on UART by our
custom WTMI image are lost.
Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I8ea4ea58e4ba3e0c0d7f47e679171b9b94442f19