OMAP clock/CPUFreq: add clk_exit_cpufreq_table()

A subsequent patch adds code on OMAP2xxx to dynamically allocate the
CPUFreq frequency table in clk_init_cpufreq_table(), so for it to
avoid a leak, it will need a corresponding function to free the
memory.  This patch adds clk_exit_cpufreq_table() with generic
code to call a chip-specific variant inside the clockfw_lock spinlock via
struct clk_functions.

Signed-off-by: Paul Walmsley <paul@pwsan.com>