clk: mvebu: ARMADA_AP_CPU_CLK needs to select ARMADA_AP_CP_HELPER

When building arm32 allmodconfig:

ld.lld: error: undefined symbol: ap_cp_unique_name
>>> referenced by ap-cpu-clk.c
>>>               clk/mvebu/ap-cpu-clk.o:(ap_cpu_clock_probe) in archive drivers/built-in.a

ap_cp_unique_name is only compiled into the kernel image when
CONFIG_ARMADA_AP_CP_HELPER is selected (as it is not user selectable).
However, CONFIG_ARMADA_AP_CPU_CLK does not select it.

This has been a problem since the driver was added to the kernel but it
was not built before commit c318ea261749 ("cpufreq: ap806: fix cpufreq
driver needs ap cpu clk") so it was never noticed.

Fixes: f756e362

 ("clk: mvebu: add CPU clock driver for Armada 7K/8K")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://lore.kernel.org/r/20200701201128.2448427-1-natechancellor@gmail.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>