drivers/flash/flash_simulator: fix flash size calculation

Flash size calculation was don with assumption that flash page size
is always 1 KB and flash size was parameterized with such granularity.

This patch correct this bug.

Flash pages number under statistic can't be calculated via preprocessor
anymore - thus are parameterized via Kconfig.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>