[ARM] 5513/1: Eurotech VIPER SBC: fix compilation error

Compilation for this board yields the following errors:

arch/arm/mach-pxa/viper.c:511: error: 'FFUART' undeclared here (not in a function)
arch/arm/mach-pxa/viper.c:520: error: 'BTUART' undeclared here (not in a function)
arch/arm/mach-pxa/viper.c:529: error: 'STUART' undeclared here (not in a function)

Fix them by including the necessary header.

Signed-off-by: Ricardo Martins <rasm@fe.up.pt>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>