clk: ti: Check kzalloc() for failures

smatch reports a failure to check kzalloc() here:

drivers/clk/ti/clk.c:232
omap2_clk_provider_init() error: potential null dereference 'io'.
(kzalloc returns null)

Check for an allocation failure and return -ENOMEM.

Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>