Thermal: spear_thermal: convert to devm_ioremap_resource

Use the newly introduced devm_ioremap_resource().

devm_ioremap_resource() provides its own error messages; so all explicit
error messages can be removed from the failure code paths.

CC: Vincenzo Frascino <vincenzo.frascino@st.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>