arm: common: Make timer configuration conditional
arm: common: Make timer configuration conditional

Make the timer configuration conditional on the optional interface being
available.

Signed-off-by: Soren Brinkmann <f9793267bd24439886cd4a142f9642a9bcc6364b@xilinx.com>

sandrine-bailleux/arm-trusted-firmware,sandrine-bailleux-arm/arm-trusted-firmware,achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,sbranden/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,sbranden/arm-trusted-firmware,achingupta/arm-trusted-firmware,sandrine-bailleux/arm-trusted-firmware,sandrine-bailleux-arm/arm-trusted-firmware