ARM: display5: Increase the pre-relocation malloc pool size to 4KiB

Porting more DTS code from Linux kernel for display5 board required
increase of pre-relocation malloc pool size in U-Boot proper.

The early malloc memory is necessary for handling parsing and setup of
e.g. serial port device (and all its ancestors in DT tree).

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Peng Fan <peng.fan@nxp.com>