atmel: Drop README.at91-soc

This issue covered by this doc appears to be fixed, so let's remove the
README.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: York Sun <york.sun@nxp.com>
Acked-by: Andreas Bießmann <andreas@biessmann.org>

doc/README.at91-soc

- New C structure AT91 SoC access
-=================================
-
-The goal
---------
-
-Currently the at91 arch uses hundreds of address defines and special
-at91_xxxx_write/read functions to access the SOC.
-The u-boot project perferred method is to access memory mapped hw
-regisister via a c structure.
-
-e.g. old
-
-	*AT91C_PIOA_IDR = AT91_PMX_AA_TWD | AT91_PMX_AA_TWCK;
-	*AT91C_PIOC_PUDR = AT91_PMX_AA_TWD | AT91_PMX_AA_TWCK;
-	*AT91C_PIOC_PER = AT91_PMX_AA_TWD | AT91_PMX_AA_TWCK;
-	*AT91C_PIOC_OER = AT91_PMX_AA_TWD | AT91_PMX_AA_TWCK;
-	*AT91C_PIOC_PIO = AT91_PMX_AA_TWD | AT91_PMX_AA_TWCK;
-
-	at91_sys_write(AT91_RSTC_CR,
-		AT91_RSTC_KEY | AT91_RSTC_PROCRST | AT91_RSTC_PERRST);
-
-e.g new
-	pin = AT91_PMX_AA_TWD | AT91_PMX_AA_TWCK;
-	writel(pin, &pio->pioa.idr);
-	writel(pin, &pio->pioa.pudr);
-	writel(pin, &pio->pioa.per);
-	writel(pin, &pio->pioa.oer);
-	writel(pin, &pio->pioa.sodr);
-
-	writel(AT91_RSTC_KEY | AT91_RSTC_CR_PROCRST |
-		AT91_RSTC_CR_PERRST, &rstc->cr);
-
-The method for updating
-------------------------
-
-1. add's the temporary CONFIG_AT91_LEGACY to all at91 board configs
-2. Display a compile time warning, if the board has not been converted
-3. add new structures for SoC access
-4. Convert arch, driver and boards file to new SoC
-5. remove legacy code, if all boards and drives are ready
-
-2013-10-30 Andreas Bießmann <andreas@biessmann.org>:
-
-The goal is almost reached, we could remove the CONFIG_AT91_LEGACY switch but
-remain the CONFIG_ATMEL_LEGACY switch until the GPIO disaster is fixed. The
-AT91 spi driver has also some CONFIG_ATMEL_LEGACY stuff left, so another point
-to fix until this README can be removed.