thermal: qoriq: Fix a compiling issue

Qoriq thermal driver is used by both PowerPC and ARM architecture.
When built for PowerPC architecture, it reports error:
undefined reference to `.__devm_regmap_init_mmio_clk'
To fix it, select config REGMAP_MMIO.

Fixes: 4316237b (thermal: qoriq: Convert driver to use regmap API)
Signed-off-by: Yuantian Tang <andy.tang@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200303084641.35687-1-andy.tang@nxp.com