dm: omap_timer: Fix conversion of address to a pointer

OMAP timer driver directly typecasts fdt_addr_t to a pointer. This is
not strictly correct, as it gives a build warning when fdt_addr_t is u64.
So, use map_physmem for a proper typecasts.

This is inspired by commit 167efe01 ("dm: ns16550: Use an address
instead of a pointer for the uart base")

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>