uboot-lantiq: danube: fix hanging lzma kernel uncompression

At least since gcc 7.3.0 (OpenWrt 18.06) lwr/lwl are used in the
assembly of LzmaProps_Decode. While the decission made by the compiler
looks perfect fine, it triggers some obscure hang on lantiq danube-s
v1.5 with MX29LV640EB NOR flash chips.

Only if the offset 1 is used, the hang can be observed. Using any other
offset works fine:

  lwl s0,0(a1) - s0 == 0x6d000080
  lwl s0,1(a1) - hangs
  lwl s0,2(a1) - s0 == 0x0080xxxx
  lwl s0,3(a1) - s0 == 0x80xxxxxx

It isn't clear whether it is a limitation of the flash chip, the EBU or
something else.

Force 8bit reads to prevent gcc optimizing the read with lwr/lwl
instructions.

Signed-off-by: Mathias Kresin <dev@kresin.me>