When the board is booted without serial cable attached (which
is how most of them will be used) UART RX is left floating and
sometimes picks noise, which interrupts countdown and enters
U-Boot prompt instead of booting the kernel.

Fix this by setting up internal pullup on UART RX pin. This
does not prevent serial from working as the internal pullup
is weak.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>