nvmem: Broaden the selection of NVMEM_SNVS_LPGPR

The SNVS LPGR IP block is also found on other i.MX SoCs that
are not covered by the current SOC_IMX6 || SOC_IMX7D logic.

One example is the i.MX7ULP.

To avoid keep expanding the SoC logic selection, make it broader
by using the more generic ARCH_MXC symbol instead.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>