asrock/e350m1: reduce default stack size

The stack used on the ASRock E350M1 is significantly less than
what we currently set (64k per core). In fact, we use about half
of the default stack size (4k) on core 0 and even less on non
BSP cores [1]:

    $ grep stack coreboot_without_patch_but_monotonic_timer.log
    CPU1: stack_base 002a0000, stack_end 002afff8
    CPU1: stack: 002a0000 - 002b0000, lowest used address 002afda8, stack used: 600 bytes
    CPU0: stack: 002b0000 - 002c0000, lowest used address 002bf75c, stack used: 2212 bytes

Removing the Kconfig variable STACK_SIZE to use the default results
in the following numbers of stack usage.

    $ grep stack coreboot_with_patch.log
    CPU1: stack_base 00287000, stack_end 00287ff8
    CPU1: stack: 00287000 - 00288000, lowest used address 00287da8, stack used: 600 bytes
    CPU0: stack: 00288000 - 00289000, lowest used address 0028875c, stack used: 2212 bytes

[1] http://review.coreboot.org/#/c/3154/
    (comment May 2 10:21 AM)

Change-Id: Ibdb2102c86094fce3787e3b5a162ca8423de205c
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Tested-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/3209
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/mainboard/asrock/e350m1/Kconfig

@@ -84,10 +84,6 @@ config HEAP_SIZE
         hex
         default 0xc0000
 
-config STACK_SIZE
-        hex
-        default 0x10000
-
 config RAMBASE
         hex
         default 0x200000