ARM: at91: fix nommu build regression

The newly introduced support for SAMA5D4 added access to the
'AT91_ALT_BASE_SYS' register area, but failed to define the
symbols in the case when CONFIG_MMU is disabled.

We really should not hardwire addresses like this any more,
but as a small fixup, this patch just adds the missing
definitions for the nommu case, which gets at91x40_defconfig
and any configuration of sam9 and sama5 with MMU disabled
back to work.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 726d32bf ("ARM: at91: SAMA5D4 SoC detection code and low ...")