arm: nrf52: Introduce NRF52 SoC Specific config options

Introduce SoC specific config options similar to what exists on NRF51,
this is mostly to help distinguish between SRAM & Flash sizes on
different variants.

Also deleted some unnecessary setting of CONFIG_SOC_NRF528{32,40} in the
board defconfig files.

Change-Id: I3aaedf0c15423ae12636f87b8e6a39070cbb2c6f
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>