Kconfig: enlarge CONFIG_SYS_MALLOC_F_LEN

"alloc space exhausted" happens in very early stage, which could be seen
with DEBUG_UART options enabled and leeds to an non-functional board.

kontron_pitx_imx8m:
CONFIG_DEBUG_UART_BASE=0x30880000   # for serial3
CONFIG_DEBUG_UART_CLOCK=24000000

imx8mqevk:
CONFIG_DEBUG_UART_BASE=0x30860000      # for uart1
CONFIG_DEBUG_UART_CLOCK=24000000

It is because CONFIG_SYS_MALLOC_F_LEN is too small and still leave
CONFIG_SPL_SYS_MALLOC_F_LEN as 0x2000.

Reported-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Heiko Thiery <heiko.thiery@gmail.com>
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>