bus: ti-sysc: Fix handling of invalid clocks

We can currently get "Unable to handle kernel paging request at
virtual address" for invalid clocks with dts node but no driver:

(__clk_get_hw) from [<c0138ebc>] (ti_sysc_find_one_clockdomain+0x18/0x34)
(ti_sysc_find_one_clockdomain) from [<c0138f0c>] (ti_sysc_clkdm_init+0x34/0xdc)
(ti_sysc_clkdm_init) from [<c0584660>] (sysc_probe+0xa50/0x10e8)
(sysc_probe) from [<c065c6ac>] (platform_drv_probe+0x58/0xa8)

Let's add IS_ERR checks to ti_sysc_clkdm_init() as And let's start treating
clk_get() with -ENOENT as a proper error. If the clock name is specified
in device tree we must succeed with clk_get() to continue. For modules with
no clock names specified in device tree we will just ignore the clocks.

Fixes: 2b2f7def ("bus: ti-sysc: Add support for missing clockdomain handling")
Acked-by: Roger Quadros <rogerq@ti.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>