dts: stm32wb: Limit flash size to 812K

stm32wb55rg is a dual core SoC (M4/M0) with a radio/protocol stack
running on M0 core. M0 FW starts at flash address 0x80CB000.
So flash size available to zephyr is 812K instead of 1M.
Configure package size to reflect this (flash size is now 812K)
and update MPU setting to exclude M0 FW. From MPU point of view,
we set flash size to 512K since we should chose a size being a power
of 2.
If we consider there will be 2 images partitions in a FOTA scheme,
512 K is beyond image-0 and hence is sufficient to cover application
image.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>