Fix AT32F405 flash size/map

[ALTracer]

Detailed description

• The F_SIZE readout and usage as implemented in Feature: Add support for Arterytek MCU AT32F405, F423 #1949 is incomplete and results in failure to reprogram internal flash of AT32F405RC.
• This PR solves that by factoring in the kibibytes multiplier, for both AT32F402/405 and AT32F423 added in that commit.

For a (C) 256 KiB flash part, the F_SIZE "register" contains not 262144 but 256, which (as used in PR1949 26d27b9) generates a target flash map of 256 byte length and 2048 (or 1024) byte erase-sectors, which is broken.

Assumed tested on some BMD-derivative platform.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (see Building the firmware)
• It builds as BMDA (see Building the BMDA)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

[dragonmux]

LGTM, merging. Thank you for the contribution and this fix!