ARM: at91: clock: Add missing include of asm/io.h

ARM: at91: clock: Add missing include of asm/io.h

In one inline function in this header `readl()` is used, but the
declaration was not found, so buildman gave this warning:

    w+include/asm/arch/clk.h: In function 'get_h32mxdiv':
    w+include/asm/arch/clk.h:65:16: warning: implicit declaration of function 'readl' [-Wimplicit-function-declaration]

Fixes: 927b901b47a6 ("ARM: atmel: add sama5d4ek board support")
Signed-off-by: Alexander Dahl <ada@thorsis.com>