firmware: turris-mox-rwtm: Use macro constant instead of hardcoded 4096

firmware: turris-mox-rwtm: Use macro constant instead of hardcoded 4096

The 4096 bytes limit in mox_hwrng_read() is due to the DMA buffer being
allocated to one PAGE_SIZE bytes. Use new local macro constant
RWTM_DMA_BUFFER_SIZE at allocation time and when used in mox_hwrng_read().

Use SZ_4K instead of PAGE_SIZE. Although PAGE_SIZE is never set to a
larger value on Armada 3720, it theoretically could, and this would be a
waste of space.

Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20240831092050.23093-2-kabel@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>