clk: hisilicon: add hisi_clk_alloc function.

Before, there was an ordering issue that the clock provider
had been published in hisi_clk_init before it could provide
valid clocks to consumers. hisi_clk_alloc is just used to
allocate memory space for struct hisi_clock_data. It makes
it possible to publish the provider after the clocks are ready.

Signed-off-by: Jiancheng Xue <xuejiancheng@hisilicon.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>