arm: lpc32xx: switch SPL builds to driver model

For NXP LPC32xx boards the change enables SPL_DM option, this allows
to use any driver model UART driver in SPL images, hence a restriction
on HSUART in SPL image is removed and well as definitions for non-DM
NS16550 driver, its DM version is used instead.

Because SPL_DM requires malloc(), enable CONFIG_SPL_SYS_MALLOC_SIMPLE
for LPC32xx boards.

The change adds about 5KB to the resulting SPL image (for devkit3250
board SPL image is increased from 10672 to 15608 bytes).

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>