riscv: mpfs-icicle-kit: Increase SYS_MALLOC_F_LEN

The RISC-V architecture default value of CONFIG_SYS_MALLOC_F_LEN
(0x1000) would not provide enough memory for devices like mpfs
clock and ns16550 serial to bind well before relocation.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Padmarao Begari <padmarao.begari@microchip.com>
Tested-by: Padmarao Begari <padmarao.begari@microchip.com>