Use double backticks instead of quotes for sysfs attribute value.
This makes sphinx generate the "mcu" and "cpu" values in monospace when
rendering to HTML.

Fixes: dfa556e4 ("platform: cznic: turris-omnia-mcu: Add support for MCU connected GPIOs")
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20240730144924.25552-3-kabel@kernel.orgSigned-off-by: Arnd Bergmann <arnd@arndb.de>