x86: qemu-x86_defconfig: Increase CONFIG_SYS_MALLOC_F_LEN

x86: qemu-x86_defconfig: Increase CONFIG_SYS_MALLOC_F_LEN

With the upcoming increase of the malloc area in U-Boot
("pci: pci-uclass: Dynamically allocate the PCI regions"), the CI QEMU
x86 test fails:

U-Boot 2020.10-rc2-g0a668f6d38 (Aug 25 2020 - 06:12:51 +0000)

alloc space exhausted
Error binding driver 'cpu_qemu': -12
Some drivers failed to bind
alloc space exhausted
initcall sequence fff6a760 failed at call fff13b3d (err=-19)

This patch now increases CONFIG_SYS_MALLOC_F_LEN to 0x1000, which is
already used on qemu-x86_64_defconfig.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>