SPL: Do not enable SPL_SYS_MALLOC_SIMPLE without SPL_FRAMEWORK by default

On P2020 board is SPL malloc simple always failing with error and loops:

  SD boot...
  alloc space exhausted
  Bad trap at PC: f8f8b5f0, SR: 21200, vector=d00
  NIP: 00000000 XER: 00000000 LR: 00000000 REGS: f8f8b5f0 TRAP: 20000000 DAR: 00000000
  MSR: 00021200 EE: 0 PR: 0 FP: 0 ME: 1 IR/DR: 00

  GPR00: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
  GPR08: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
  GPR16: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
  GPR24: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
  Call backtrace:
  Exception in kernel pc f8f8b5f0 signal 0

Inspection showed that gd->malloc_limit is zero. And it is because
generally SPL_FRAMEWORK initialize SPL's gd->malloc_limit. But when
SPL_FRAMEWORK is not enabled then in most cases nobody initialize
gd->malloc_limit and so SPL malloc simple does not work.

So disable SPL_SYS_MALLOC_SIMPLE by default when SPL_FRAMEWORK is not
enabled. SPL_SYS_MALLOC_SIMPLE can be disabled only by setting
SPL_SYS_MALLOC_F_LEN to zero. So do it.

This change fixes SPL error "alloc space exhausted" on P2020 board.

Signed-off-by: Pali Rohár <pali@kernel.org>