arm: beetle: Fix UART1 IRQ number

The commit de78ecd7 "arm: beetle: Use device tree for IRQs" introduces
a regression related to UART1 IRQ.

On arm beetle the UART1 has IRQ number 2.

This patch restores the functionality modifying the arm beetle device
tree file.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>