arm: stm32: Limit u-boot.itb size to 0x160000 bytes on DH STM32MP15 DHSOM

The maximum size of u-boot.itb in SPI NOR on DH STM32MP15 DHSOM is
0x160000 . Define this size in U-Boot config to prevent misconfigured
builds from emitting larger u-boot.itb than the one which fits the
SPI NOR area reserved for the blob.

The SPI NOR layout is as follows:
0x00_0000..0x03_ffff ... SPL 1
0x04_0000..0x07_ffff ... SPL 2
0x08_0000..0x1d_ffff ... U-Boot
0x1e_0000..0x1e_ffff ... Environment 1
0x1f_0000..0x1f_ffff ... Environment 2

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>