ARM: da850evm: Increase Malloc Size

The SPI NOR frameware was recently updated, but they require a little
more memory that we had previosly allocated, so it stopped booting.
This patch doubles the size of CONFIG_SPL_SYS_MALLOC_F_LEN to 0x800.

Fixes: c4e88623 ("mtd: spi: Switch to new SPI NOR framework")

Signed-off-by: Adam Ford <aford173@gmail.com>