Kconfig: Enlarge default SYS_MALLOC_F_LEN for AM33XX

Since commit 8e14ba7b ("gpio: omap_gpio: Add DM_FLAG_PRE_RELOC
flag") omap GPIO gets bound before relocation.  Unfortunately due to
this, on at least the beaglebone black, the pre-relocation memory pool
gets exhausted before probing the serial port. This then causes u-boot
to panic as CONFIG_REQUIRE_SERIAL_CONSOLE is set...

Resolve this by resizing the default size of the pre-relocation malloc
pool for AM335X platforms.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>