stm32mp: stm32prog: use defines for virtual partition size

Use the existing defines PMIC_SIZE and OTP_SIZE and a new define
CMD_SIZE for virtual partition size.

This patch corrects the size for OTP partition in alternate name
(1024 instead of 512) and avoids other alignment issues.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>