clk: keystone: gate: fix error handling on init

This patch fixes Keystone gate control clock driver initialization path:
1) clk_register_psc() returns error code and not a pure pointer, hence
its return value need to be checked using IS_ERR(clk) macro.

2) Mapped IO memory isn't unmapped in case of errors, hence fix it.

Cc: Mike Turquette <mturquette@linaro.org
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>