Kconfigs: layerscape: use a convenient default value for SYS_MALLOC_F_LEN

The default value of CONFIG_SYS_MALLOC_F_LEN (0x400)
leaves U-Boot with not enough memory to load i2c driver
before relocate, causing it to hang.

Change the default value of CONFIG_SYS_MALLOC_F_LEN
for below SoCs,
- LS1012A
- LS1021A
- LS1043A
- LS1046A

Signed-off-by: Biwen Li <biwen.li@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>