ram: stm32mp1: protect minimum value in get_bufsize

Add protection on minimum value for result of get_bufsize
and check the alignment of buffer size: only multiple min_size
is allowed; only 4 bytes alignment was checked previously
(value & 0x3).

For example the "Random" test raises an issue when size is not 8 bytes
aligned because address for buffer = address + size / 2 is not word
aligned.

This patch avoid test error for unsupported size value.

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