ddb: fix show devmap output on 32-bit arm

The output has been broken since 1b6dd6d772ca. Casting to uintmax_t
before the call to printf is necessary to ensure that 32-bit addresses
are interpreted correctly.

PR: 243236
Approved by: re (gjb)

(cherry picked from commit 9976b42b697ce203b1d257b2a6fe64c8a2961645)
(cherry picked from commit 0a223cf980b5ef8bea726a843fe5a7faf2278330)