arm: actions: increase SYS_MALLOC_F_LEN

after commit 4ab3817f ("clk: fixed-rate: Enable DM_FLAG_PRE_RELOC flag")
Cubieboard7 (based on actions S700 SoC) fails to boot.

It is due to the fact that the default value of CONFIG_SYS_MALLOC_F_LEN (0x400)
would not provide enough memory for clock device to probe (before relocation)
well.

This commit fixes it, by increasing SYS_MALLOC_F_LEN to value 0x2000.

Suggested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>