rockchip: spi: rk_spi: use base variable with uintptr_t size

The fdt_addr_t and phys_addr_t size have been decoupled.
A 32bit CPU can expect 64-bit data from the device tree parser,
so use a base variable with uintptr_t size in the
rk_spi.c file.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>