doc: platform: cznic: turris-omnia-mcu: Use double backticks for attribute value

doc: platform: cznic: turris-omnia-mcu: Use double backticks for attribute value

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: dfa556e45ae9 ("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.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>