clk: bulk: add of_clk_bulk_get()

'clock-names' property is optional in DT, so of_clk_bulk_get() is
introduced here to handle this for DT users without 'clock-names'
specified. Later clk_bulk_get_all() will be implemented on top of
it and this API will be kept private until someone proves they need
it because they don't have a struct device pointer.

Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Russell King <linux@arm.linux.org.uk>
Reported-by: Shawn Guo <shawnguo@kernel.org>
Tested-by: Thor Thayer <thor.thayer@linux.intel.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>