fs: nvs: move `FLASH_PAGE_LAYOUT` to `depends on`

`FLASH_PAGE_LAYOUT` has a hardware dependency on `FLASH_HAS_PAGE_LAYOUT`
which is not present for all boards. Forcing this symbol to `y` when
the hardware doesn't support it results in build errors at the Kconfig
stage.

`FLASH_PAGE_LAYOUT` is enabled by default when `FLASH_HAS_PAGE_LAYOUT`
is true, so this change will not require any user changes.

Signed-off-by: Jordan Yates <jordan@embeint.com>