spl: do not repeat timer init on i.MX6

The GPT timer was already initialised in board_init_f() as
it is needed in dram init. Do not repeat timer init in
board_init_r().

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>