ARM: mach-at91: Add compile time option to choose proper timer

New SoC's of AT91 family with ARM-9 core includes a regular timer and a 64-bit
timer.This patch adds a compile time option to the Makefile such that the old
timer driver is chosen and compiled as default if none of timer configuration
options are explicitly defined in the board configs.

Signed-off-by: Hari Prasath <Hari.PrasathGE@microchip.com>
Reviewed-by: Eugen Hristev <eugen.hristev@microchip.com>