ram: stm32mp1: use the DDR size by default in the test addressBus

ram: stm32mp1: use the DDR size by default in the test addressBus

The test 4 = "AddressBus [size] [addr]" without parameter
detects alias for any address bit only when:
- size = real size of DDR
- addr = start of DDR = 0xC0000000

These value must be the default value when parameters are absent.

This patch sets bufsize to STM32_DDR_SIZE and get_bufsize() selects
the correct value for bufsize when this parameter is absent =
full size of the DDDR

On EV1 board :
DDR> test 4
running at 0xC0000000 length 0x40000000

On DK2 board
DDR> test 4
running at 0xC0000000 length 0x20000000

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