riscv: Add place-holder asm/arch/clk.h for driver compilation

Some of the drivers (such as Cadence MACB ethernet driver) expect
asm/arch/clk.h to be provided by arch support so we add place-holder
asm/arch-generic/clk.h for RISC-V generic CPU.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>