drivers: core: use strcmp when find device by name

`if (!strncmp(dev->name, name, strlen(name)))` might find out
the wrong device, it might find out `dram_pll_ref_sel`, when name is
`dram_pll`. So use strcmp to avoid such issue.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>