imx: variscite: Fix regression of SPL and U-boot booting

1. Commit 3a7c45f6 ("simple-bus: add DM_FLAG_PRE_RELOC flag to
simple-bus driver") causes some i.MX boards that were converted
to DM, such as warp7, to fail to boot.

As explained by Lukas Auer:

"With the patch, U-Boot probes the drivers for devices under simple-bus
device tree nodes in the pre-relocation device model. The default value
of CONFIG_SYS_MALLOC_F_LEN (0x4000) leaves U-Boot with not enough memory to
do this, causing it to hang."

Fix this problem by providing a convenient default value for
CONFIG_SYS_MALLOC_F_LEN.

2. CONFIG_SPL_TEXT_BASE was moved to Kconfig, so added in defconfig
"configs: move CONFIG_SPL_TEXT_BASE to Kconfig"
(sha1: f89d6133)

Signed-off-by: Saravanan Sekar <saravanan@linumiz.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>