sunxi: arm64: Increase CONFIG_SYS_BOOTM_LEN to 32MB

The default value of CONFIG_SYS_BOOTM_LEN, 0x800000, causes error
when uncompressing Image.gz out of FIT image.

  Uncompressing Kernel Image ... Error: inflate() returned -5
Image too large: increase CONFIG_SYS_BOOTM_LEN

and loading Image out of FIT image.
   Loading Kernel Image ... Image too large: increase CONFIG_SYS_BOOTM_LEN
Must RESET board to recover

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>