imx8/ls10xx: Use a sane SYS_MALLOC_F_LEN default

When adding new features to imx8m boards, such as DM clock support,
the malloc area can be exhausted.

To avoid such issue, provide a reasonable default for the
SYS_MALLOC_F_LEN size.

Quoting Tom Rini:

"This seems to be an area where everyone is either:
- Kicking the value up a bit for themselves
- Having hard to figure out problems booting the platform because it's
  too small a value until they see someone else picked a larger value.

So lets raise these a bit and get some acks, please."

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Tested-by: Adam Ford <aford173@gmail.com> #imx8mm-beacon, imx8mn-beacon
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>