doc: services: nvmem: Mention flash configuration

Add an entry to the configuration section for flash related Kconfig
symbols.

Signed-off-by: Pieter De Gendt <[email protected]>

Author: pdgendt

doc/services/nvmem/index.rst

@@ -34,6 +34,7 @@ Configuration
 
 * :kconfig:option:`CONFIG_NVMEM`: Enables the NVMEM subsystem.
 * :kconfig:option:`CONFIG_NVMEM_EEPROM`: Enables NVMEM support for EEPROM devices.
+* :kconfig:option-regex:`CONFIG_NVMEM_FLASH.*`: Configure NVMEM support for flash devices.
 
 Devicetree Bindings
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