make the CPU clock selectable via Kconfig

this removes the sunxi specific CONFIG_CLK_FULL_SPEED defined in each
soc header and replaces it's use in board/sunxi/board.c with
CONFIG_SYS_CLK_FREQ from Kconfig which allows us to configure board
specific frequency on boot

Signed-off-by: Iain Paton <ipaton0@gmail.com>
[hdegoede@redhat.com s/CONFIG_SYS_CLK_FREQ/CONFIG_TIMER_CLK_FREQ/ for the
 arch-timer clk speed on sun7i to fix mis-compile on sun7i]
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>